How Can I Solve This Errror??

Hi, I am trying to update my table... but when i try to save my query.. i am getting the following error

 Column 'SourceDBF.DEFPCT' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause. This is my sproc...

 

 UPDATE
Statements..ParticipantSourceSummary
SET
DeferralPct = DEFPCT,
BeginingBal = BEGBAL,
Deposits = DEPOSITS,
Withdraw = WITHDRAW,
GainLoss = GAINLOSS,
Others = OTHER,
EndingBal = ENDBALANCE,
VestPercent = VESTPCT,
VestBal = VESTBALANC
FROM
Statements..ParticipantSourceSummary ps
Join (
SELECT
p.ParticipantId,
cp.PlanId,
cs.SourceId,
@PeriodId PeriodId,
s.DEFPCT,
s.BEGBAL,
s.DEPOSITS,
s.WITHDRAW,
s.GAINLOSS,
s.OTHER,
s.ENDBALANCE,
s.VESTPCT,
s.VESTBALANC
FROM
SourceDBF s
INNER JOIN Statements..ClientPlan cp
ON s.PLAN_NUM = cp.ClientPlanId
INNER JOIN Statements..ClientSources cs
ON s.SOURCE_NUM = cs.ClientSourceId and cs.ClientId = @ClientId
Inner Join Statements..Participant p
on s.PART_ID = p.SSN
Where
s.Import = 1
GROUP BY
p.ParticipantId,
cp.PlanId,
cs.SourceId
) s
ONps.ParticipantId = s.ParticipantId
AND
ps.PlanId = s.PlanId
AND
ps.SourceId = s.SourceId
AND
ps.PeriodId = s.PeriodId  any help will be appreciated.RegardsKaren