Moving Average -> Standard Deviation

[rsbin]

Good Morning,

Reaching out to the DAX experts out there.  I am trying to calculate the Standard Deviation of a Measure based on a 3 Day Moving Average (i.e. 3DayStdDev_EquipIDRatio).  My sample data looks like this:

Date	GSIndex	EquipmentIDCount	TotalVisits	EquipmentIDRatio
8/1/2019	1	3079	13555	0.227
8/2/2019	2	2931	13524	0.217
8/3/2019	3	1037	4997	0.208
8/4/2019	4	637	3326	0.192
8/5/2019	5	2326	8619	0.270
8/6/2019	6	2908	14599	0.199
8/7/2019	7	2994	14383	0.208

Date	GSIndex	3DayEquipID	3DayVisits	3DayStdDev_EquipIDRatio	Std. Dev Excel
8/1/2019	1
8/2/2019	2
8/3/2019	3	7047	32076	0	0.0080
8/4/2019	4	4605	21847	0	0.0104
8/5/2019	5	4000	16942	0	0.0338
8/6/2019	6	5871	26544	0	0.0353
8/7/2019	7	8228	37601	0	0.0314

EquipIDCount , TotalVisits and Equipment IDRatio are all Measures.

EquipIDRatio is a measure  - simply (EquipIDCount / TotalVisits).

3DayVisits and 3DayEquipID are moving average calculations wherein I use the Index to Sumx the 3 values (Current Day +2 prior days). 

I am able to calculate Standard Deviation on my database as a whole using this DAX provided by @Greg_Deckler and @az38 

StdDev_EquipIDRatio = 
VAR _table = SUMMARIZE(FILTER(GeneralStatistics, GeneralStatistics[TotalVisits] <> 0),[Date],"_EquipIDRatio",[EquipmentIDRatio])
RETURN STDEVX.P(_table,[_EquipIDRatio])

The Standard Deviation in the last column was calculated in Excel.  This is the number I am trying to replicate in DAX.  This number is the Standard Deviation of the EquipmentIDRatio of the current day + the previous 2 days.

So on day 3, I need to take the Ratio values from Day 1 - 3

On day 4, I need to take the Ratio values from Day 2 - 4. And so on.....

Since EquipIDRatio is a measure, to calculate Std Dev.,  I need to construct a virtual table as I have done above, but containing only the 3 relevant values for each Date or Index.  I have hit a wall and unable to figure out the correct way to do this in DAX.

Hoping one of you can point me in the right direction.

Thanks in advance and best regards,

[Icey]

Hi @rsbin ,

How about this?

3DayStdDev_EquipIDRatio =
VAR CurrentIndex =
    MAX ( GeneralStatistics[GSIndex] )
VAR PreviousIndex = CurrentIndex - 2
VAR _table =
    SUMMARIZE (
        FILTER (
            GeneralStatistics,
            GeneralStatistics[TotalVisits] <> 0
                && GeneralStatistics[GSIndex] >= PreviousIndex
                && GeneralStatistics[GSIndex] <= CurrentIndex
        ),
        [Date],
        "_EquipIDRatio", [EquipmentIDRatio]
    )
RETURN
    STDEVX.P ( _table, [_EquipIDRatio] )

Best Regards,

Icey

If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.

[rsbin]

Good Morning @Icey 

Thanks much for the reply.   I have attempted that code as as Solution, but it returns only an answer for the last date.

The ".03142" is the correct answer for Index = 7.   I too thought it would cycle through and provide an answer for each Index, but as you can from the above, it Returns just a single value.

I got up this morning thinking "CALCULATETABLE" might be the way to proceed.   Really appreciate you taking the time to respond. If you have further suggestions I would be grateful to hear them.  Kudos for the attempt.

Kind Regards,

[Icey · Accepted solution]

Hi @rsbin ,

How about using ALLSELECTED in the expression?

3DayStdDev_EquipIDRatio =
VAR CurrentIndex =
    MAX ( GeneralStatistics[GSIndex] )
VAR PreviousIndex = CurrentIndex - 2
VAR _table =
    SUMMARIZE (
        FILTER (
            ALLSELECTED (GeneralStatistics ),   ------------------edited
            GeneralStatistics[TotalVisits] <> 0
                && GeneralStatistics[GSIndex] >= PreviousIndex
                && GeneralStatistics[GSIndex] <= CurrentIndex
        ),
        [Date],
        "_EquipIDRatio", [EquipmentIDRatio]
    )
RETURN
    STDEVX.P ( _table, [_EquipIDRatio] )

Best Regards,

Icey

If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.

[rsbin]

Good Morning @Icey 

Yes, the edit works.  I finally am able to get the Std. Dev values for each Index.   Thank you very much for the assistance.

Unfortunately, still having some issues having it display properly in my line charts.   I have to figure out why it is behaving like that.

But thanks again, it is a relief to finally solve the first part of this.

Kind Regards and all the best,