more than 2 groups found, only 2 allowed

[Abdullah Akram]

I have only 2 groups defined by "1" & "0", but I am getting the following error, when applying the Z test.

error- more than 2 groups found, only 2 allowed

command used for ztest : ztest arlocation, by(arlocation) sd1(117) sd2(18.42)

any help would be much appreciated, thanks!

[Nick Cox]

Check for missing values and other values

tab arlocation, missing

[Hemanshu Kumar]

Perhaps you also have missing values in the grouping variable?

Also surely the variable you want to compare means of, cannot be the same as the variable that defines the groups?

ztest arlocation, by(arlocation) sd1(117) sd2(18.42)

Edit: crossed with #2

[Abdullah Akram]

Perhaps you also have missing values in the grouping variable?

Also surely the variable you want to compare means of, cannot be the same as the variable that defines the groups?

ztest arlocation, by(arlocation) sd1(117) sd2(18.42)

Edit: crossed with #2

hey sorry it was a typo its "by(arlocationc)"

and I do not have any missing value, I rechecked but I am still getting the same error

[Abdullah Akram]

Check for missing values and other values

tab arlocation, missing

Hi thanks for the reply, I do not have any missing value.

[Abdullah Akram]

hey guys thanks for helping me out. I just figured it out.
instead of using numeric numbers, "1" & "0" for defining variables, I used the string variable now its working.

[Leonardo Guizzetti]

Your by group must have 2 distinct values, or else you can use the -if- qualifier to restrict it to two categories. Missing values do not count towards the by-group levels and ar marked out of the estimation sample.

hey guys thanks for helping me out. I just figured it out.
instead of using numeric numbers, "1" & "0" for defining variables, I used the string variable now its working.

That should not have solved your problem, so I would be suspicious of your results.

Here's a toy example showing these points

sysuse auto, clear

// using if qualifier, we only select 2 levels of rep78 to use for the test.
cap noi ztest price if inlist(rep78, 1, 3), by(rep78)

cap noi ztest price, by(rep78) // gives error

recode rep78 (1/2=1) (3/5=0), gen(rep2grp)
tab rep2grp, mi
ztest price, by(rep2grp) // no error

gen rep2grp_string = string(rep2grp) if !mi(rep2grp)
ztest price, by(rep2grp_string) // no error