Standard Deviation as a column either in Power Query or DAX

Hi satkin - I am assuming your table is named standardObservations and your columns are date, Variables, and Values

create a dax measure as follows and before that I hope you have a seperate date table created in your model. if not please create it.

Measure:

StdDevLast6Months =
VAR CurrentDate = MAX('standardObservations'[Date])
VAR StartDate = EDATE(CurrentDate, -6)
RETURN
CALCULATE(
STDEV.P('standardObservations'[Values]),
FILTER(
'standardObservations',
'standardObservations'[Date] >= StartDate &&
'standardObservations'[Date] <= CurrentDate &&
'standardObservations'[Variable] = MAX('Observations'[Variable])
)
)

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satkin although rajendraongole1  has given the solution, as a best practice, add a date dimension in your model and use it for time intelligence calculations. Once the date dimension is added, mark it as a date table on table tools. Check the related videos on my YT channel

Add Date Dimension
Importance of Date Dimension
Mark date dimension as a date table - why and how?
Time Intelligence Playlist

and then I will write a DAX measure like this:

StdDevLast6Months =
CALCULATE(
STDEV.P('standardObservations'[Values]),
DATESINPERIOD ( 'Calendar'[Date], MAX ( 'Calendar'[Date] ), -6, MONTH )
)