Stored Procedures Subquery Returns More Than One Value?

SQL (using 2012) .I'm trying to create some stored procedures that calculate stats needed for item response theory and test scoring (things like reliability, correlation, etc.). I'm writing a procedure that's aimed at splitting a test in half (first half of the items and second half of the items) and calculating each individual (TestID) score for each half. I can insert the first half of the test, but end up getting an error stating that the subquery returned more than one value. I know it's being caused by the group by clause, but if I remove it I end up with a total count of the items answered correctly, but it's necessary for me to link it batch to the TestID (so each row should have a BatchID [same for all at the moment], a unique TestId, a corresponding FirstHalfScore and a corresponding SecondHalfScore for each testID).

Insert into SplitHalfTestScores (BatchId, TestID, FirstHalfScore)
(Select distinct 10001 as BatchId, irt.TestID, count(*) as FirstHalfScore
From irt_TestItems irt where batchID=10001 and ItemId <=40 and TestId = irt.TestId and AnsweredCorrectly = 1 group by TestId)

Update SplitHalfTestScores
Set SecondHalfScore = (Select count(*)
From irt_TestItems irt where BatchId = 10001 and TestId = irt.TestId and irt.ItemId > 40 and AnsweredCorrectly = 1
group by TestId)