问题
My data and code are like this:
my_vector <- rnorm(150)
my_factor1 <- gl(3,50)
my_factor2 <- gl(2,75)
tapply(my_vector, my_factor1, function(x)
t.test(my_vector~my_factor2, paired=T))
I want to do a separate t-test for each level of my_factor1, to test my_vector for both levels of my_factor2.
However, with my code the t-test is not splitting the levels of my_factor1, and the results are equal for each level because my_vector is entirely included in each t.test.
This is the output of my code:
$`1`
Paired t-test
data: my_vector by my_factor2
t = 0.2448, df = 74, p-value = 0.8073
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-0.2866512 0.3669667
sample estimates:
mean of the differences
0.04015775
$`2`
Paired t-test
data: my_vector by my_factor2
t = 0.2448, df = 74, p-value = 0.8073
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-0.2866512 0.3669667
sample estimates:
mean of the differences
0.04015775
$`3`
Paired t-test
data: my_vector by my_factor2
t = 0.2448, df = 74, p-value = 0.8073
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-0.2866512 0.3669667
sample estimates:
mean of the differences
0.04015775
What am I missing or doing wrong?
回答1:
Your example is slightly problematic, since if you set:
df <- data.frame(my_vector = rnorm(150),
my_factor1 = gl(3,50),
my_factor2 = gl(2,75)
)
You will have only one unique value for my_factor2
when my_factor1
= 1 or 3 because of how your repetitions overlap. See ?gl
. So do:
df <- data.frame(my_vector = rnorm(150),
my_factor1 = gl(3,1,150),
my_factor2 = gl(2,1,150)
)
with(df,
by(df, my_factor1,
function(x) t.test(my_vector ~ my_factor2, data=x)
)
)
Which appears to produce your desired output.
As a side note-- consider correction for multiple comparisons: https://stats.stackexchange.com/questions/16779/when-is-multiple-comparison-correction-necessary
来源:https://stackoverflow.com/questions/30332752/how-to-perform-t-tests-for-each-level-of-a-factor-with-tapply