# ~/Blog

## Brandon Rozek

PhD Student @ RPI studying Automated Reasoning in AI and Linux Enthusiast.

# Uniformity of Math.random()

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There are many cases where websites use random number generators to influence some sort of page behavior. One test to ensure the quality of a random number generator is to see if after many cases, the numbers produced follow a uniform distribution.

Today, I will compare Internet Explorer 11, Chrome, and Firefox on a Windows 7 machine and report my results.

## Hypothesis

H0: The random numbers outputted follow the uniform distribution

HA: The random numbers outputted do not follow the uniform distribution

## Gathering Data

I wrote a small website and obtained my data by getting the CSV outputted when I use IE11, Firefox, and Chrome.

The website works by producing a random number using Math.random() between 1 and 1000 inclusive and calls the function 1,000,000 times. Storing it’s results in a file

This website produces a file with all the numbers separated by a comma. We want these commas to be replaced by newlines. To do so, we can run a simple command in the terminal

``````grep -oE '[0-9]+' Random.csv &gt; Random_corrected.csv
``````

Do this with all three files and make sure to keep track of which is which.

Here are a copy of my files for Firefox, Chrome, and IE11

## Check Conditions

Since we’re interested in if the random values occur uniformly, we need to perform a Chi-Square test for Goodness of Fit. With every test comes some assumptions

Counted Data Condition: The data can be converted from quantatative to count data.

Independence Assumption: One random value does not affect another.

Expected Cell Frequency Condition: The expected counts are going to be 10000

Since all of the conditions are met, we can use the Chi-square test of Goodness of Fit

## Descriptive Statistics

For the rest of the article, we will use R for analysis. Looking at the histograms for the three browsers below. The random numbers all appear to occur uniformly

``````rm(list=ls())
``````
``````hist(ie11\$V1, main = "Distribution of Random Values for IE11", xlab = "Random Value")
``````

``````hist(firefox\$V1, main = "Distribution of Random Values for Firefox", xlab = "Random Value")
``````

``````hist(chrome\$V1, main = "Distribution of Random Values for Chrome", xlab = "Random Value")
``````

## Chi-Square Test

Before we run our test, we need to convert the quantatative data to count data by using the plyr package

``````#Transform to count data
library(plyr)
chrome_count = count(chrome)
firefox_count = count(firefox)
ie11_count = count(ie11)
``````

Run the tests

``````# Chi-Square Test for Goodness-of-Fit
chrome_test = chisq.test(chrome_count\$freq)
firefox_test = chisq.test(firefox_count\$freq)
ie11_test = chisq.test(ie11_count\$freq)

# Test results
chrome_test
``````

As you can see in the test results below, we fail to reject the null hypothesis at a 5% significance level because all of the p-values are above 0.05.

``````##
##  Chi-squared test for given probabilities
##
## data:  chrome_count\$freq
## X-squared = 101.67, df = 99, p-value = 0.4069
``````

`firefox_test`

``````##
##  Chi-squared test for given probabilities
##
## data:  firefox_count\$freq
## X-squared = 105.15, df = 99, p-value = 0.3172
``````

`ie11_test`

``````##
##  Chi-squared test for given probabilities
##
## data:  ie11_count\$freq
## X-squared = 78.285, df = 99, p-value = 0.9384
``````

## Conclusion

At a 5% significance level, we fail to obtain enough evidence to suggest that the distribution of random number is not uniform. This is a good thing since it shows us that our random number generators give all numbers an equal chance of being represented. We can use `Math.random()` with ease of mind.