Stats with R - 1

Source DataSet :- https://spark-public.s3.amazonaws.com/stats1/datafiles/Stats1.13.HW.02.txt

Initially I read the text file in form of a dataframe in R by running the following command ,I named the dataframe as "impact" :-

> impact <- read.table("Stats1.13.HW.02.txt",header = T)

A data set with 96 obsetvations and 4 varibles is created in R

1.How many rows of data are in the data file?
96
Make sure you read the data with the argument: header=TRUE Then: nrow(data) OR dim(data)

2.What is the name of the dependent variable?
command : names(data)
Ans : SR 

3.What is the mean of SR across all subjects? 
command : mean(data$SR) OR describe(data)

> mean(impact$SR)
[1] 12.65625


4.What is the variance of SR across all subjects?
Command : var(data$SR

output :
> var(impact$SR)
[1] 6.54375

5.What is the mean of SR for all subjects at pretest?
command :- pre = subset(data, data$time=="pre") THEN mean(pre$SR)

Output :
> pre<- subset(impact,impact[ ,3]== "pre")
> mean(pre$SR)
[1] 12.02083

6.What is the standard deviation of SR for all subjects at posttest?
command :- post = subset(data, data$time=="post") THEN sd(post$SR)

Output :
> sd(post$SR)
[1] 2.449128

7.What is the median of SR for all subjects at posttest?
describe(post$SR) AND median(post$SR)

Output:-
> describe(post$SR) 
  var  n  mean   sd  median trimmed  mad min max range skew kurtosis   se
1   1 48 13.29  2.45   13.5   13.28     2.22   9  19    10     0.06    -0.49    0.35
> median(post$SR)
[1] 13.5

8.Which group has the highest mean at posttest?
Ans :- DS
.Command :- describeBy(post, post$condition)

Output :-

9.Which one best approximates a normal distribution?
 Option :- 

WM group at pretest
WM group at postest    Ans :- WM group at postest	Correct
PE group at pretest
PE group at posttest
DS group at pretest
DS group at postest Code :-

pre.wm = subset(pre, pre$condition=="WM")
post.wm = subset(post, post$condition=="WM")
pre.pe = subset(pre, pre$condition=="PE")
post.pe = subset(post, post$condition=="PE")
pre.ds = subset(pre, pre$condition=="DS")
post.ds = subset(post, post$condition=="DS")
par(mfrow= c(2,3))
hist(pre.wm[,4])
hist(post.wm[,4])
hist(pre.pe[,4])
hist(post.pe[,4])
hist(pre.ds[,4])
hist(post.ds[,4])

10.Which group showed the biggest gains in SR?
Code :-
pre.wm = subset(pre, pre$condition=="WM")
post.wm = subset(post, post$condition=="WM")
pre.pe = subset(pre, pre$condition=="PE")
post.pe = subset(post, post$condition=="PE")
pre.ds = subset(pre, pre$condition=="DS")
post.ds = subset(post, post$condition=="DS")
> mean(post.wm$SR)-mean(pre.wm$SR) 
[1] 1.3125
> mean(post.pe$SR)-mean(pre.pe$SR) 
[1] 0.0625
> mean(post.ds$SR)-mean(pre.ds$SR)
[1] 2.4375

ANS :- DS