Standard deviation over a rolling 6 months

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Standard deviation over a rolling 6 months

I'm trying to calculate the standard deviation over a rolling 6 months.  The code that I am using isn't giving me the correct calculation and I don't understand why.  Below is the code, can someone help me understand what I am doing wrong and how to correct it?

cast(stddev_pop(indexgoal) over (partition by agency, fa order by fa, rpt_month rows 5 preceding) as decimal(12,5)) as stdIndGoal

Thanks,

CDW