
---
title:  "STAT340: Discussion 13: Multiple Testing"
author: "Names"
date:   "`r format(Sys.time(), '%d %B, %Y')`" # autogenerate date as date of last knit
documentclass: article
classoption: letterpaper
output:
  html_document:
    highlight: tango
    fig_caption: false
---

```{r setup, include=FALSE}
# if sourced, set working directory to file location
# added tryCatch in case knitting runs into error
tryCatch({
  if(Sys.getenv('RSTUDIO')=='1'){
    setwd(dirname(rstudioapi::getActiveDocumentContext()$path))
  }}, error = function(e){}
)

# install necessary packages
if(!require(pacman)) install.packages("pacman")
pacman::p_load(knitr,tidyverse)

knitr::opts_chunk$set(tidy=FALSE,strip.white=FALSE,fig.align="center",comment=" #")
options(width=120)
```

---

<style>
a:link {text-decoration: underline;}
h2{margin-top:40px;}
h3{margin-top:30px;}
h4{margin-top:30px;font-style: italic;}
</style>



[Link to source file](ds13.Rmd)



## Actually relevant XKCD comic (3 min)

<center><a href="https://xkcd.com/882/"><img id="comic" src="https://imgs.xkcd.com/comics/significant.png" title="'So, uh, we did the green study again and got no link. It was probably a--' 'RESEARCH CONFLICTED ON GREEN JELLY BEAN/ACNE LINK; MORE STUDY RECOMMENDED!'"></a></center>




## Discuss together (~5 min)

As an entire section, discuss these together:

- What does multiple testing mean, why is it a problem, and how do methods like Bonferroni work?




## Exercises

We will explore the problem of multiple testing in 2 exercises. First, we will run a Monte Carlo simulation study to quantitatively demonstrate the problem of multiple testing. Then, we will apply our knowledge to a real data set.




### 1) Simulation

For this exercise, we will **randomly generate data** that's completely independent, and show that with enough comparisons, we can easily **produce falsely significant results**. We will also show how Bonferroni's correction can mitigate these problems.

#### Generating data

Let `nvar` represent the **number of predictor variables** to be generated. To begin, let `nvar=20` (which corresponds to the number of variables you would expect to need to generate on average to produce 1 falsely significant predictor at the standard α=0.05 level (note this is [geometric](https://karlrohe.github.io/340-Spring21/text/chapter_01_random_variables/random_variables.html))).

Also let `nobs` be the **number of observations** we take (i.e. number of rows in our data frame). Let's begin with `nobs=1000`.

```{r}
nvar = 20
nobs = 1000
```

Next, we randomly generate a data frame with `nobs` rows and `nvar+1` columns (1 observed variable + `nvar` predictor variables). The distribution doesn't matter, let's just start by using the standard normal distribution.

The easiest way to do this is to generate a vector with `nobs*(nvar+1)` random numbers, then reshape to a matrix with the correct dimensions, then finally convert to a data frame object (note matrix and data frame are two different types of objects in R, so this last step is necessary or else the R functions we are used to using (which are written for data frames) will not work).

```{r}
# UNCOMMENT LINES BELOW AND RUN

# df = rnorm(nobs*(nvar+1)) %>%       # generate random numbesr
#   matrix(ncol = nvar+1) %>%         # reshape to matrix with nvar+1 columns
#   as.data.frame %>%                 # convert to data frame
#   setNames(c("Y",paste0("X",1:20))) # change column names
# 
# str(df)
# head(df,4)
```

#### Running regression

Remember, **multiple testing correction methods are just methods for adjusting p-values** to take into account how many tests we are running, so it can apply to any statistical method that produces a p-value; it is NOT limited to regression. However, for this example, we will use regression to demonstrate the problem.

Run a basic multiple linear regression (no interactions or powers) of `Y` on all variables `X1`, `X2`, ..., `X20` in the dataset (recall you the expression `Y ~ .` will do this automagically (don't forget to set `data=df`)), and show the `summary()` output.

```{r}
# run regression here

```

The numbers in your dataset are all randomly and independently generated, but due to the number of tests you are running, on average you would find 1 statistically significant (p-value < 0.05) variable in this summary output (**if you didn't get a significant result, try running the data generation again**).

#### Monte-Carlo analysis

Recall that a p-value represents the probability of **false positive** if there is **no real effect**. Thus, letting α=0.05 means we expect on average, 5\% of our experiments to produce a false positive.

Write a Monte-Carlo function that, for each iteration, generates a new data frame using the method above, performs a multiple linear regression, and checks to see if the results report any falsely significant variables. (**Hint:** you can use `coef(summary( ... ))[,4]` to extract the computed p-values from your `lm` model object).

```{r}
# write Monte-Carlo function here.
# at this point of the course, you should be
# able to write this with minimal guidance.


```

Run the function `N=1000` times. What proportion of these 'experiments' produced some kind of falsely significant variable?

 > _**RESPONSE TEXT HERE**_

#### Bonferroni correction

Now, apply a Bonferroni correction. Note **there are two equivalent method** of doing this; you can **either**

 - use α=0.05/20=0.0025 instead of the uncorrected α=0.05,
 - OR multiply the p-value for each test by 20 and compare with the uncorrected α=0.05.

Modify your previous function (ideally by adding an additional TRUE/FALSE argument) so that each iteration, a Bonferroni correction is made before checking if any variables are significant. Run this new function `N=1000` times.

What proportion of these Bonferroni-corrected experiments produced a falsely significant variable? Is this consistent with what you expect? (**Hint:** remember you expect on average α=0.05 experiments to produce false positives.)

 > _**RESPONSE TEXT HERE**_

#### BONUS section

If you wish, rerun the above results (i.e. proportion of uncorrected and Bonferroni-corrected experiments with a false positive variable in the results) for different values of `nvar` and plot the two curves, along with a flat line at 0.05. What do you observe?

<br/>





### 2) Real data

For the real data analysis, we will use the `airquality` dataset (already included in R), which contains air quality measurements in New York, from May to September (5 months in total) in 1973 (see `?airquality` for more info).

Use the function `pairwise.t.test` with `x=Ozone` and `g=Month` to **run several pairwise t-tests** comparing each month to every other month (a total of 5\*4/2=10 comparisons). **Run this twice** first with the argument `p.adjust.method="none"` for no adjustment and then with `p.adjust.method="bonferroni"` and compare the results.

```{r}
# UNCOMMENT LINE BELOW TO RUN

# airquality %$% pairwise.t.test(Ozone, Month, p.adj=......)


# note that Ozone and Month are columns in airquality, so
# you have to expose the columns for pairwise.t.test to use.
# an easy way of doing this is to use %$% exposition pipe from magrittr,
# (see https://magrittr.tidyverse.org/reference/exposition.html).
# this works in a similar way to %>% except instead of piping an object,
# it exposes the columns of a data frame to be used directly in the next line.

# also note most arguments can be shortened, so
# "p.adjust.method" can be shortened to just "p.adj".
# this is possible since in R arguments are often partially matched, which means
# you only need to specify enough of it to disambiguate from other options
```

Which months are significantly different in Ozone content according to Bonferroni-corrected p-values? Which months were significant without correction but found to be NOT significant after Bonferroni correction?

 > _**RESPONSE TEXT HERE**_

<br/>




## Submission

As usual, make sure the names of everyone who worked on this with you is included in the header of this document. Then, knit this document and submit both this file and the HTML output on Canvas under Assignments ⇒ Discussion 13.

