{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "#### Let's learn the T-distribution! Note: Learn about the normal distribution first!" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "For previous distributions the sample size was assumed large (N>30). For sample sizes that are less than 30, otherwise (N<30). Note: Sometimes the t-distribution is known as the student distribution." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The t-distribution allows for use of small samples, but does so by sacrificing certainty with a margin-of-error trade-off. The t-distribution takes into account the sample size using n-1 degrees of freedom, which means there is a different t-distribution for every different sample size. If we see the t-distribution against a normal distribution, you'll notice the tail ends increase as the peak get 'squished' down. \n", "\n", "It's important to note, that as n gets larger, the t-distribution converges into a normal distribution." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "To further explain degrees of freedom and how it relates tothe t-distribution, you can think of degrees of freedom as an adjustment to the sample size, such as (n-1). This is connected to the idea that we are estimating something of a larger population, in practice it gives a slightly larger margin of error in the estimate." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Let's define a new variable called t, where\n", ":\n", "$$t=\\frac{\\overline{X}-\\mu}{s}\\sqrt{N-1}=\\frac{\\overline{X}-\\mu}{s/\\sqrt{N}}$$" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "which is analogous to the z statistic given by $$z=\\frac{\\overline{X}-\\mu}{\\sigma/\\sqrt{N}}$$" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The sampling distribution for t can be obtained:" ] }, { "cell_type": "markdown", "metadata": { "collapsed": true }, "source": [ "## $$ f(t) = \\frac {\\varGamma(\\frac{v+1}{2})}{\\sqrt{v\\pi}\\varGamma(\\frac{v}{2})} (1+\\frac{t^2}{v})^{-\\frac{v+1}{2}}$$" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Where the gamma function is: $$\\varGamma(n)=(n-1)!$$" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "And v is the number of degrees of freedom, typically equal to N-1. " ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Similar to a z-score table used with a normal distribution, a t-distribution uses a t-table. Knowing the degrees of freedom and the desired cumulative probability (e.g. P(T >= t) ) we can find the value of t. Here is an example of a lookup table for a t-distribution: " ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "http://www.sjsu.edu/faculty/gerstman/StatPrimer/t-table.pdf\n" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Now let's see how to get the t-distribution in Python using scipy!" ] }, { "cell_type": "code", "execution_count": 5, "metadata": { "collapsed": false }, "outputs": [ { "data": { "text/plain": [ "[]" ] }, "execution_count": 5, "metadata": {}, "output_type": "execute_result" }, { "data": { "image/png": [ "iVBORw0KGgoAAAANSUhEUgAAAXgAAAEACAYAAAC57G0KAAAABHNCSVQICAgIfAhkiAAAAAlwSFlz\n", "AAALEgAACxIB0t1+/AAAIABJREFUeJzt3XmUXGWd//H3hw6oLCogDBrCIgYIKkxYArLZAdTAIGF+\n", "jCIquA0iTMAZlR+O/I5kjmdGPeNxUBkwYlRUnIiyDCg72BIWIYGYsCSQgEEStrAOi0oyfH9/PLdJ\n", "penuqu6uqufWrc/rnDrJrXufrm+lu7956nufRRGBmZlVz3q5AzAzs9ZwgjczqygneDOzinKCNzOr\n", "KCd4M7OKcoI3M6uougle0jRJSyQtlXTaMNftJWmNpKNG2tbMzJpv2AQvqQc4C5gG7AIcI2nSENd9\n", 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http://stattrek.com/probability-distributions/t-distribution.aspx" ] }, { "cell_type": "code", "execution_count": null, "metadata": { "collapsed": true }, "outputs": [], "source": [] } ], "metadata": { "kernelspec": { "display_name": "Python 2", "language": "python", "name": "python2" }, "language_info": { "codemirror_mode": { "name": "ipython", "version": 2 }, "file_extension": ".py", "mimetype": "text/x-python", "name": "python", "nbconvert_exporter": "python", "pygments_lexer": "ipython2", "version": "2.7.7" } }, "nbformat": 4, "nbformat_minor": 0 }