R: how can I create a table with mean and sd according to experimental group alongside p-values?

Question

I know how I can do all that for individual variables but I need to report this information for a large number of variables and would like to know if there is an efficient w

Answer 1

First let's make some example data. For each sample, we have a unique ID, its experimental group, and some variables for which we want to calculate the mean and SD.

## Make a data frame called "Data" with five columns
Data <- as.data.frame(cbind(1:100, sample(1:2), rnorm(100), rnorm(100), rnorm(100), rnorm(100)))
names(Data) <- c("ID", "Group", "V1", "V2", "V3", "V4")

## Now we will take a peak at the top of our data frame
> head(Data)

  ID Group         V1         V2         V3         V4
1  1     2  0.3681539 -0.5008400  1.2060665 -0.7352376
2  2     1 -0.1043180  2.2038190 -1.4367898  2.1961246
3  3     2 -0.2720279 -0.5923554 -1.4628190 -1.8776453
4  4     1 -2.3299662 -0.1216227  0.4200776  1.5504020
5  5     2 -0.3670578 -1.5903221 -0.6287083 -1.0543262
6  6     1  0.4840047 -0.3181554 -1.4596980 -0.4261827

Now we can run a for loop through the variables, pull their means and SDs and p values, and dump them all in an object called "Results".

## Define object which will receive our results
Results <- NULL
Results <- as.data.frame(Results)

## Open for loop
for (i in 3:6) {

## Run the t.test() and save it in a temporary object
temp <- t.test(Data[which(Data$Group == 1), i], Data[which(Data$Group == 2), i])

## Put the name of our variable in our results object
Results[i-2,1] <- names(Data)[i]

## Group 1 mean and SD
Results[i-2,2] <- temp$estimate[1]
Results[i-2,3] <- sd(Data[which(Data$Group == 1), i])

## Group 2 mean and SD
Results[i-2,4] <- temp$estimate[2]
Results[i-2,5] <- sd(Data[which(Data$Group == 2), i])

## P value for difference
Results[i-2,6] <- temp$p.value

rm(temp)
}

Now we can make our results pretty and print them.

## Add column names
names(Results) <- c("Variable", "Group.1.Mean", "Group.1.SD", "Group.2.Mean", "Group.2.SD", "P.Value")

## View our results
> Results

  Variable Group.1.Mean Group.1.SD Group.2.Mean Group.2.SD   P.Value
1       V1   0.21544390  0.9404104  -0.01426226  1.0570324 0.2537820
2       V2   0.26287585  1.0048291   0.22992285  0.9709686 0.8679038
3       V3  -0.06112963  0.9855287   0.17423440  1.0198694 0.2434507
4       V4   0.33848678  0.9360016   0.07905932  0.9106595 0.1632705