# DAX: calculating maximum period of LSA non-compliance from individual probe readings

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The Problem

The contractor has to maintain the values of n probes under a certain threshold.
Data is measured each 5 minutes, 24 hours per day.
An incident is defined as the period while a probe readings are continuously over the threshold
LSA is based on the number of incidents in a month, and the maximum period of an incident

In the following PowerBI page we can see an incident for probe CPD_TEMP03 , when the threshold is set at level 25

Although it is trivial to know if at a certain time the value is over the threshold, it will require some tweaking with the data to know the duration of the corresponding incident. We will build the following measures:
IsIncident (boolean): when Average(value) is over the Threshold
Duration of incident (time): for an incident, time difference between last reading without incident and current reading, calculated at the original table level
Greatest duration of incident (time): for a period, the greatest value of the previous measure

We can expect performance problems, since we will have to iterate through the Temperature Readings table, for each row of that table for calculating Duration of Incident, and once again to find Greatest duration of incident. That is, Number of Rows * Number of Rows * Number of Rows. For each probe, we have up to 12values/hour * 24hours/day * 31 day/month = 8929 readings/month , which means traversing 8929 ^3 = 711.643.594.752, rather a big number 🙁

I will add some measures as follows:

Is Over Threshold = AVERAGE(‘Temperature Readings'[Value]) > Threshold[Threshold Value]

Time since tripping =
var CurrentDateTime = AVERAGE(‘Calendar'[Date/Time])
var FirstDateT =CALCULATE(min(‘Calendar'[Date/Time]), ALLSELECTED(‘Calendar’))
var LastDateT = CALCULATE(mAX(‘Calendar'[Date/Time]), ALLSELECTED(‘Calendar’))
var Readings = CALCULATE(COUNT(‘Calendar'[Date/Time]), ALLSELECTED(‘Calendar’))
var IsFirstReadingTripped = CALCULATE([Is Over Threshold],’Calendar'[Date/Time] = FirstDateT)
var interval = DIVIDE(DATEDIFF(FirstDateT,LastDateT,MINUTE), Readings -1)
var LastTimeBeforeTripping =
IF(
FirstDateT – interval/60/24,
CALCULATE(
max(‘Calendar'[Date/Time]) ,
Filter( ALL(‘Calendar’), ‘Calendar'[Date/Time] <= CurrentDateTime && CALCULATE([Is Over Threshold]) = FALSE() ) ) )
var duration = IF( HASONEVALUE(‘Calendar'[Date/Time]),
datediff(LastTimeBeforeTripping,CurrentDateTime, MINUTE)
)
return duration

Max Duration of Incident =
MAXX(
ALLSELECTED(‘Calendar’),
CALCULATE([Time since tripping])
)

For a period of 4 days, in my PC (i7, 32Gb), it takes about 2 seconds to construct the table as shown

Not so bad. Let’s see what happens when we put a month worth of data

For a week of data, perfomance drops to:

For two weeks, I get

For 28 days, even worse, as expected: Now it is not a good idea to leave the dates selector open to any range of days anymore 🙁

Can you find a better way to calculate this measure?

You may download the example file from here