# Machine Learning — Proof & Terms

# Terms

A **random variable** is a variable holding an outcome or the result of some outcomes in a random process e.g. the observed value in rolling a die, or the total sum after rolling a die 10 times. In concept, we can treat the value stored in a random variable is generated by some data distribution (say *P(x=1)=P(2)=P(3)=P(4)=P(5)=P(6)=1/6*).

**Stochastic process** or **random process** contains sequences of events with values determined by probabilities. Technically, it is modeled by an indexed collection of random variables. (usually time-indexed, like *Xt*).

The **Strong law of large numbers** states that the average of the results from a large number of trails will be close to the expected value and its difference will get smaller and smaller with more trails.

**Confounding variables**:

*X* and *Y* are confounded by a variable *Z* if *Z* causally influences both *X* and *Y*. For example, gender may influence the choice of drugs and recovery.

**Conjugate prior**:** **If the posterior distributions calculated by the Bayes’ theorem are in the same distribution family (say Gaussian distribution) as the prior, the prior is a **conjugate prior** for the likelihood function. Conjugate prior opens the possibilities of calculating the distribution for the Bayes’ Theorem result analytically.

**Parametric model vs non-parametric model**: A parametric model captures a model with equations and parameters. We use the training data to estimate the model parameters. We don’t need the training data when making an inference. A non-parametric model uses the similarity between training data and the testing data to make a prediction. The *K*-nearest neighbor classifier is a typical non-parametric model.

**Bayes’ Theorem** **terms**:

With the exception of the marginal probability, posterior, prior and likelihood are not a point estimate. It is a curve. In the example below, they show the probability density function (PDF) on different car locations.

Marginal probability normalizes the posterior to be between 0 and 1. Here are the explanations on different terms. In this example, we want to find the infection rate for flu in the current season.

**Maxwell–Boltzmann distribution**

The probability density function of a random variable at temperature *T*.

**Autoencoder**

Autoencoder composes of an encoder to discover latent factors and a decoder to reconstruct the input. By minimizing the reconstruction error, we create a model to extract latent factors and to reconstruct from it.

**One-hot vector**

Encode categorical information.

**Overfit**

**Gradient descent optimization methods (Momentum, Adagrad, RMSprop, Adam)**

Advanced optimization methods based on gradient descent.

**Sampling methods**

Simple random sample: Every data has an equal chance to be selected.

Stratified random sample:** **The data is divided into groups like male or female. Samples come from each group with members of each group selected randomly.

Cluster random sample: The data is first split into groups. Some groups are selected and we select its members randomly.

Systematic random sample: Data is ordered and select systematically.

**Selection bias**

Selection bias is the bias introduced by the selection of data.

- Sampling bias: Non-random sampling of the population. For example, in self-selection, responders to a survey may not be a good representation of the general population.
- Pre-screening: Randomness can be skewed after pre-screening.
- Time interval: For example, the annual stock returns during a recession is not a good indicator of the general stock returns.
- Attrition: A selection bias caused by attrition (loss of participants). For example, patients may exit a study because of its ineffectiveness. The final result may only include those react better with the treatment.
- Susceptibility bias: An example from Wikipedia — postmenopausal syndrome gives a higher likelihood of also developing endometrial cancer, so estrogens given for the postmenopausal syndrome may receive a higher than actual blame for causing endometrial cancer.
- Data: Accept (like cherry-picking) or reject a specific set of data.
- Studies: Perform repeated experiments and report only the most favorable results.

**Survivorship bias**

This bias is caused by the overlook of samples, say because of the lack of visibility or no longer exist.

**Autoregression**

Use the previous timestep state(s) as input(s) to solve a regression problem.

**Gram matrices**

**Box plot & Scatter plot**

**Exploratory data analysis**

Understand the data insights and relationships through visualization or statistical analysis. The following is a **Multivariate analysis in **which each box varies two variables at a time.

**Box Cox Transformation**

A Box Cox transformation transforms a non-normal dependent variables into a normal shape.

With the following equations:

**Categorical Variable**

A variable contains discrete values, e.g. (apple, orange, …, peach)

**Lasso Regression**

Lasso regression is a linear regression using L1 regularization and squared errors.

**Activation function**

Sigmoid, ReLU, leaky ReLU and tanh in creating non-linearity of a model:

**Softplus function**

Here is a plot for ReLU and Softplus function.

**Swish**

Swish is an activation function experimentally designed by performing a search with reinforcement learning. Its objective is to find a function that can maximize the accuracy (reward) for multiple DL models (like classification with ResNet).

**Bucketing**

Encode a continuous variable with buckets.

**AUC (Area under curve)**

Find the area under a curve.

**Spectral theorem**

Every *n* × *n* symmetric matrix ** S** has

*n*real eigenvalues

*λᵢ*with

*n*chosen orthonormal eigenvectors

*vᵢ*.

These eigenvalues can form a diagonal matrix *Λ *as *diag(λ). *We can also concatenate eigenvectors *vᵢ* to form *V. *i.e.,

We rename *V* as *Q. *Because *Q* is orthogonal, it is invertible and *Qᵀ = Q⁻ ¹*. Therefore, the symmetric matrix ** S** can be factorized into

This is the Spectral theorem on how to diagonalized a symmetric matrix.

**Confusion matrix**

A confusion matrix holds the values of true positive, false positive, true negative and false negative.

**independently and identically distributed i.i.d.**

Data points are drawn from a distribution that doesn’t change, and each drawn value does not depend on others.

**pseudorandom**

Computer random number generator is not actually random. Given all the seeds known, it creates a known and deterministic sequence of numbers. We call this pseudorandom because its statistics are not distinguishable from a truly random number generator.

**logits**

The ratio *p*/(*1-p*) is called odds. logits (log odds) takes the log of odds.

**Covariates, measurement, features, inputs, independent variables, dependent variable, response, output, prediction**

We call the input *x *to a model *f(x) c*ovariates, measurement, features, inputs, or independent variables. We call the output dependent variable, response, prediction or output.

**Covariate shift**

For example, we train a model for the housing price in San Francisco to predict the housing price for Beverley Hill. The predictions will not be accurate due to the covariate shift. (a.k.a. a shift in the data distribution between the data in training and inference.)

**Generative classifier & Gaussian Discriminant Analysis**

**Cross-validation**

We divide the sample into *K* groups. We use *K-1* groups to train the model and one to collect validation performance. We rotate that held-out group for *k* times and aggregate the performance results from the hold-out groups to access how good the model is. In particular, how good the model can be generalized and not overfitted. We use this method to evaluate the gap between the training error and the validation error. Or we can use that to select hyperparameters. Nevertheless, it is less popular in DL when we prefer to collect a larger dataset and use some part of it dedicated to validation and testing.

**Perceptron algorithm**

Perceptron uses the gradient descent method to learn a classifier assuming all data points are linearly separable into 2 classes.

Here are the cost function and gradient.

**Convexity**

Inside a shape, if it is convex, any points between two points fall inside the shape also.

**Primal and dual problems**

Primal problem:

Lagrange dual problem:

Given a Lagrangian, the Lagrange dual function and the Lagrange dual problem are:

**Hard clustering & soft clustering**

In clustering, the use of a probability in evaluating the cluster assignment for each data point is called the soft clustering. Alternatively, we can assign a data point to a cluster deterministically in each training iteration. This is called hard clustering. Soft clustering usually has a smoother cost function that makes training easier and more stable.

**Sample space, Event space**

The sample space Ω for rolling dice is {1, 2, 3, 4, 5, 6}. *p*(*ω*) is the probability for each outcome *ω*. For a fair dice, *p*(*ω=*1) = 1/6. An event is a subset of the sample space (*A⊆Ω*). For example, *P*(*A*) = 3/6 for the event *A* {1, 3, 5}, i.e. the event when the dice value is odd.

**NP, NP-Complete & NP-Hard**

A **decision problem** is a problem which output either “yes” or “no”. For example, whether the number 13 is a prime number. This type of problem seems simple but restrictive. Nevertheless, many problems can be rephrased and solved as a series of decision problems. For example, the minimum distance of the traveling salesman problem can be rephrased as whether the salesman can travel all cities less than *N*. Then starting from a small value of N, we can try different values until the decision problem returns true.

NP (nondeterministic polynomial time) is the set of decision problems which if we are provided with a solution, we can verify the answer in polynomial time. For example, given the number 91, is it a prime number? This is an NP problem. But given the factors 13, we can verify in polynomial time that 91 can be factored by 13 and therefore not a prime number.

NP-complete is any problem *X* in NP which any problem in NP can be transformed to *X* in polynomial time.

NP-hard is any problem X which any problem in NP can be transformed to *X* in polynomial time. NP-hard does not need to be an NP problem or a decision problem. It should be as hard or harder than an NP problem.

If we can solve NP-Hard or NP-complete problem in polynomial time, any NP problem can be solved in polynomial time. But it is generally believed not the case even no one can prove it yet.

**3-SAT**

3-SAT asks about the satisfiability of a logical formula defined with *n* literals *X₁*, . . . , and *Xn. *The formula contains m “AND” clauses and each clause OR together at most 3 variables.

We want to determine whether we can find some values of *X₁*, . . . , *Xn *which the formula can be evaluated to be true. 3-SAT is an NP-hard problem.

# Math

**Jensen’s inequality**

For a convex function, Jensen’s inequality is the inequality underlined in red below.

For a concave function, the inequality will be

**Domain**

The domain** **of a function is the set of input values that the function is defined.

For example *f*(*x*), *x *∈ ℝ.

**Support**

The support of *f* is the set of points in *X* that *f*(*x*) ≠ 0.

**Closed-form solution**

In general, a closed-form solution or expression is a solution or expression that can be computed with a finite number of operations. Intractable** **problems are problems that cannot be solved efficiently. For example, the complexity grows exponentially with the number of input.

# NLP

**Bag of words**

Bag of words counts the frequency of words without preserving the ordering.

**n-gram & Bigram**

*n*-gram is a contiguous sequence of *n* items from a given text (an ordered sequence of *n* words.) and bigram is a 2-gram.

**Skip-gram model and Continuous Bag-of-Words model (CBOW)**

Skip-gram model predicts each context word from its target word. CBOW is the opposite which we predict the target word from its context.

**Negative sampling**

**Part-of-speech (POS) tagging**

**Named Entity Recognition**

# Proof

**Optimal model parameters for linear regression with MSE**

**Sample variance**

If data *x* is Gaussian distributed with limited samples, the tradition equation for variance is a biased estimation for *σ²*.

To correct the bias, we introduce a new **estimator** for the variance, the **sample variance**:

**Variance in estimating Bernoulli Distribution**

**Poisson Distribution**

**Logistic loss**

In logistic regression, we compute the probability of being a class *Y* as

**Gaussian process with Cholesky decomposition**

## Probability ratio for logistic regression

Logistic regression

This is the Hinge loss and we have proven it from the perspective of probability ratio and constraint violation.

**Bayes’ theorem**

Sometimes the result of the Bayes’ theorem can surprise you. Let’s say there is an uncommon genetic disease that only 0.1% of the population has it. We develop a test of 99% accuracy for the positive and the negative result. i.e. if 100 tests are done on people with the disease, 99 of them will be positive. If 100 tests are done on people without the disease, only 1 will be positive. What is the chance that you have this disease if your test positive? The answer is only 9% which is much smaller than you think. The intuition is that even the test looks accurate, it generates more false positive than true positive because the disease is not common. Therefore, for those tested positive, most of them are false alarms. With Bayes’ theorem, we can demonstrate that it is only 9% of having the disease even if you are tested positive.

Notation: *d* stands for having the disease and + stands for testing positive. ¬*d* means not d.

**Bayesian inference example**

Let’s determine the infection rate (*θ*) of flu for this season. On the first day of the flu season, we got 10 Flu testing results back. We call this **evidence** (*x*). (It is often called observation also.) Our new evidence shows that 4 samples are positive. (*x*=4 out of 10). We can conclude that (*θ=4/10=0.4*). Nevertheless, we want to study further by calculating the probability of our evidence given a specific *θ*, i.e. (*p*(*x*|*θ*) which is called the **likelihood** of our evidence. Here is the equation.

Here is the plot against *θ*.

Frequentist makes an inference based on evidence. Bayesian inference combines belief with evidence. If we have collected many sample data, the evidence is strong and the inference will resemble the sampled result. Otherwise, we depend on a prior belief in guiding us before more evidence coming in.

In our example, we had collected 100 months of data on the infection rate for flu. This forms a prior belief on the infectious rate for flu in general — but not the particular strain for this season. In the first iteration, we start with a strong prior based on the past 100 months of information. In the first iteration where new sample data is rare, the posterior will resemble the prior closer.

But as more sample data is collected, the likelihood becomes more certain and the posterior get closer to the likelihood.

**Bayes’ Theorem application**

There are hidden states of a system that cannot observe directly or easily. Personally, I get some good idea on the chance that I feel happy and sad on average. I also know reasonably well on what I may do when I am happy or sad. On the contrary, the chance of feeling happy given I watch a movie is harder to determine. Here is the information that we collect.

As demonstrated by the Bayes’ Theorem, there is a 94% chance that I feel happy given I went to a party.

**HMM**

Find the probability of the current state *xt* given all the observations *y*.

**RBM & Free energy**

Free energy *F* is defined as

Maximize the log-likelihood with the free energy *F*:

**MSE is composed of bias and variance**

We are ignoring ε in modeling *y = f(x) + ε* where ε is the zero-center Gaussian distributed noise with variance *σ²*. If we don’t ignore it, the expected value for the MSE is

**Variance**

**Mean & Variance for MLE estimator**

(The proof here utilize the proof in the last section.)

**Optimization with constraint(s) — solved by Lagrange multiplier**

**Minimum L2 Regression**

Given

*w_in* above is a solution for *Xw = y*. We want to prove that it also satisfies

i.e. w_in has the lowest L2-norm for any solution for *y* = *Xw*.

(Proof is originated from here.)

**Tower rule**

**c4.5 tree**

This algorithm has a few base cases.

- All the samples in the branch belong to the same class. Create a leaf node for it.
- No information gain from any features. C4.5 creates a decision node higher up the tree.

The full algorithm

**SVM maximize the margin**

Before the proof, we need to understand convexity and the decision boundary first in the next two sections. (For simplicity, we will make claims about convexity and decision boundary without proof here.) Also, the proof here assumes the data points can be completely separated into the corresponding classes.

**Convexity representation**

Inside a shape, if it is convex, any points between two points fall inside the shape also.

Mathematically, any points inside a convex shape can be represented as a linear combination of other training data points with the conditions below.

**Decision hyperplane**

The decision hyperplane is always orthogonal to ** w **for linear regression

*wᵀx*.

**SVM maximize margin proof**

Let’s assume we have two convex shapes that form the boundary of two different classes. For example, the blue pentagon encloses all data points belong to the blue class.

Now, we want to find *u* and *v*, the supporting vectors that have the shortest distance among any other possible pairs between these two classes. From the previous section, we show *w *is always perpendicular to the margin boundary. As shown below, by simple geometry, to have the maximum margin cutting through the supporting vectors, the optimal *w* *that has the maximum margin ‖ m*‖ must be lying on *uv*.

Therefore, if we prove the optimal *w* in the SVM objective is along the line *uv, *we can prove SVM maximize the margin. (The equations and proof here are originated from Professor Paisley Course.)

Let’s define the supporting vectors first (two shortest distance points between two classes). It can be written as finding point *u* and *v* in the following objective using the convexity concept.

The SVM objective is mathematically defined as

We can apply the Lagrange Multiplier to solve the optimization problem with the Lagrangian 𝓛 defined as.

where *xᵢ* is the training data point *i*. To optimize the solution, the Lagrange Multiplier involves two steps. First, minimize 𝓛 w.r.t. *w* and *w₀ *(the primal problem)*. *We substitute the solution into 𝓛 to create a dual problem that we want to maximize w.r.t. *αᵢ*. (more information on Lagrange Multiplier)

For the first minimization, we set the corresponding derivative w.r.t. to *w* and *w₀* to 0.

After minimizing the Lagrangian, we apply the solution to 𝓛.

After the substitution, the dual problem (the objective that we want to maximize next) becomes

These can be solved by a software package to find the optimized *αᵢ*.

After solving *αᵢ, *we can use condition ① to solve *w*.

Next, we want to solve *w₀. *Since our second optimization step maximizes 𝓛 w.r.t. *αᵢ*, the optimized *αᵢ *should be greater than 0 only if *yᵢ*(*xᵢᵀw + w₀*)-1=0.

So for any *αᵢ > *0, we can solve *w₀ *by solving *yᵢ*(*xᵢᵀw + w₀*)-1=0.

Let’s revisit condition ②. The sum of *αᵢ *(*C*)* *from the same class is equal to other class.

We can split the terms below based on the label values which is either +1 or -1 and then divide it by *C*.

We plug both results into the dual problem.

We get

We realize that maximizing the dual problem is similar to minimizing the term underline in red below.

Notice that this is the same objective in finding the supporting vectors. Let summarize this. We start with the SVM objective function. We use Lagrange Multiplier to solve the problem. It turns out the optimization problem is the same as finding the distance between the supporting vectors — the minimum separation between 2 class boundary. So their optimal solutions fit well with both objectives.

Finally, we need to visualize *w* and see why SVM maximizes the margin.

i.e. *w* actual lay on the line *uv*. Since *w* is always orthogonal to the decision hyperplane, the corresponding margin boundary will be the maximum. As shown below, if *w* is not aligned with *uv, *the corresponding margin will be smaller.

**SVM with violation**

In practice, due to noise, not all data points are easily separable into the corresponding classes.

We introduce *ξᵢ* here to have some allowed marginal error for each data point. The SVM objective becomes.

The dual problem becomes (it use 𝜙(*x*) instead of *x* to expand the concept even further).

The optimized solution for the primal problem is

Plug in the solution to the Lagrangian. The dual problem that we want to solve becomes (We further expand the concept to include Kernel *K*)

The major difference from our previous solution is the additional constraint *αᵢ* ≤ λ. Most of the *αᵢ *will be zero. For *αᵢ > *0*, *those corresponding to the support vectors on the boundary.

To make predictions,

**AdaBoost error upper bound**

(Credit: the equation and the proof is originated from this source).

The upper bound for the training error in AdaBoost is

The proof involves steps to prove each inequality for the condition a ≤ b ≤ c below.

First, let’s review a few equations and definition. In each iteration, we use *w*(*i*) to determine the chance of picking data point *i*. ** Z** is the normalization factor for

*w*(such that all

*wᵢ*add up to one).

We expand the equation for *w* and define ** b**.

And establish the condition

Next, we want to prove the first inequality a ≤ b.

Next, we want to prove b ≤ c.

Now we can show *a* ≤ *c* which is what we want to prove.

**EM (Proof 1)**

**EM (Proof 2)**

**Solving missing data with EM algorithm**

(Credit: the equation and the proof is originated from this source).

Consider data *xᵢ* that have a different number of missing elements at different locations.

We can use the EM algorithm to find the missing data as well as *μ *and Σ that model *x*.

In the E-step, we find a method to compute *q *with *p*(*θ₂|x, θ₁*)

Now, we know how to compute *q. *We can integrate over *x*ᵐᵢ to compute the log-likelihood *p*(*xᵒᵢ *| *μ , *Σ).

We can optimize the log-likelihood in the M-step as

Given the following definitions

The optimized *μ *and Σ can be computed as:

**GMM with EM algorithm**

(Credit: the equation and the proof is originated from this source).

Let’s define a model on how data is generated in GMM. For example, say 0.4 chance that the new data belongs to cluster A and 0.6 chance that it belongs to cluster B. Then π = {0.4, 0.6}. Once we have the cluster assignment *cᵢ*, we sample *xᵢ* from the corresponding Gaussian distribution of the cluster *cᵢ*.

We want to learn the model *θ₁* with the following parameters.

Let’s start with the log likelihood again in the EM algorithm which is expressed as

and *θ*₂ is the cluster assignment c*ᵢ.*

Set *q*(*cᵢ = k*) to *p*(*cᵢ = k | xᵢ, π, μ, Σ*) in the E-step

In the M-step, we cross out the denominator that is constant relative to *π, μ, Σ, *and the second term which *q = p* and therefore it is zero.

So the log-likelihood that we want to optimize is:

We can differentiate the objective above w.r.t. *π, μ, *Σ to find the optimal *π, μ, *Σ*. *Here are the solution and the final algorithm.

**Matrix factorizing**

(Credit: the equation and the proof is originated from this source).

Let’s define the notation for the latent factors for user *i* and item *j*. Mᵢⱼ contains a valid rating (non-empty entry).

** U** contains are user latent factors and

**contains all item latent factors. The user rating matrix is**

*V**R*=

*UVᵀ*.

We solve it by

But *u* depends on *v* and vice versa. So we cannot solve it analytically. But we can solve it in gradient descent in alternating steps.

Here is the final algorithm:

**PCA with EM algorithm**

We model *X≈WZ*. Here is our generative model for *X* and *z*. We are going to learn *W* and *z*.

However, it is intractable if we solve it with MLE.

Instead, we will apply the EM algorithm.

Without proof, it is the solution.

**Bayesian Information Criterion**

(Credit: the equation and the proof is originated from this source).

In BIC, we want to maximize the posterior of the model. After applying the Bayes’ Theorem, we remove any terms that are constant w.r.t. *Mᵢ*.

The integral above is hard to compute. But we can estimate it with the Laplace approximation. Apply Laplace approximation to optimize the objective.

Laplace approximation approximates *ln* p(*D|M*) as a Gaussian distribution. So the integral below actually equals the normalization factor in a Gaussian distribution.

Recall the definition for 𝓙 and apply the i.i.d. assumption, we get

Plug the value back and we arrive at the BIC penalty.

**Conjugate Prior (Dirichlet distribution)**

Dirichlet distribution is the conjugate prior of multinomial:

The posterior predictive is:

The marginal is:

**Importance sampling**

With importance sampling, the optimal choice for *q* in lowering the variance of the expectation estimation is

Given

① is the importance sampling with *q*. Therefore, the expectation of *f* over *p* is the same as the importance sampling with *q*, i.e.* i*mportance sampling is unbiased regardless of the choice of *q* as long as *q*(*x*) >0 when *p*(*x*) > 0.

We can write the expectation value as *μ *which is independent of* q*. Let’s expand the variance for E[*f(x)p(x)/q*(x)*] on the R.H.S. for *q**.

So the lower bound for the variance of any *q* is the variance of *q**, i.e. *q** has the lowest variance.