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## Control chart for binary (yes/no) variable

Good morning,

I work in Quality Improvement and control charts are something we use everyday. Creating a control chart for continuous data is quite simple but I am having tremendous difficulty in creating a control chart for a binary variable (e.g. a process: pass or fail). I need to track pass rate overtime (daily, weekly, monthly, yearly) with Upper and Lower Control Limits.

Any feedback or insight in this matter would be highly appreciated.

Estefania

7 REPLIES 7
Frequent Visitor

Thank you for replying and I apologize for not providing enough detail. @Anonymous @Greg_Deckler @amitchandak @DataZoe

Control charts have 4 componenets, the count of the measure (average), the central line (the average of the data for a prior time period- serves as a baseline) , and upper and lower control limits (+/- 3 sigma line).

My data does look like this:

SampleID | Date            | Outcome

1              | 2020-01-01 | 1

2              | 2020-01-01 | 0

3              | 2020-01-01 | 1

What I am strugling with is creating the upper and lower control limits (the red lines in the graph below). It would be a breeze if the data was continuous but I don't know if there is an alternative given my binary variable.

Super User

@rubio3003 - Shouldn't those be:

Upper Limit = AVERAGE([Outcome]) + 3 * STDDEV('Table'[Output])

Lower Limit = AVERAGE([Outcome]) - 3 * STDDEV('Table'[Output])

Or am I missing something?

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Frequent Visitor

@Greg_Deckler  You can only calculate STDEV of continuous variables, mine is a binary variable 😞

Anonymous
Not applicable

This should not be that hard 🙂

You have to have some data first. Let's say this is what you have, more or less:

SampleID | Date            | Outcome

1              | 2020-01-01 | 1

2              | 2020-01-01 | 0

3              | 2020-01-01 | 1

...

You get the idea. Now, the above is your fact table where you record all the outcomes at the lowest granularity. You also have to have a Date table that will join to Samples[Date]. Date is your date/time dimension. In this table you'll define all the pieces of time you want: months, years, weeks... you name it.

Now, here's a measure that will give you the % of passes/successes:

[Success Rate] = divide( sum( Samples[Outcome] ), countrows( Samples ) )

[Failure Rate] = 1 - [Success Rate]

And that's basically it. The column Outcome should store 0 for fail, 1 for success.

Super User

@rubio3003 - Are you referring to this: https://en.wikipedia.org/wiki/Control_chart

I would use 1 and 0 to represent your binary states. Other than that, Not really enough information to go on, please first check if your issue is a common issue listed here: https://community.powerbi.com/t5/Community-Blog/Before-You-Post-Read-This/ba-p/1116882

The most important parts are:
1. Sample data as text, use the table tool in the editing bar
2. Expected output from sample data
3. Explanation in words of how to get from 1. to 2.

Become an expert!: Enterprise DNA
External Tools: MSHGQM
Latest book!:
The Definitive Guide to Power Query (M)

DAX is easy, CALCULATE makes DAX hard...
Super User

pass% = divide(calculate(countrows(Table) ,[process]="pass") ,countrows(Table) )

fail%= divide(calculate(countrows(Table) ,[process]="fail") ,countrows(Table) )

Create a date table for analysis

To get the best of the time intelligence function. Make sure you have a date calendar and it has been marked as the date in model view. Also, join it with the date column of your fact/s. Refer :
https://www.archerpoint.com/blog/Posts/creating-date-table-power-bi
https://www.sqlbi.com/articles/creating-a-simple-date-table-in-dax/

See if my webinar on Time Intelligence can help: https://community.powerbi.com/t5/Webinars-and-Video-Gallery/PowerBI-Time-Intelligence-Calendar-WTD-Y...

Employee

@rubio3003 I am not very familiar with control charts but I did a quick search and I think I have the idea. If this isn't it, would it be possible to share a small table of what you the data looks like and even a drawing of what the chart is you are trying to create?  Thanks!

I do think for scatter plots in power bi you need measures, which may be the issue here?

For your pass/fail, this can converted to a percentage with:

Things = countrows('Table')

Pass Percentage = divide(caclulate([Things]),'Table'[Pass/Fail]="Pass"),[Things])

now to aggregate by different ranges, you could do something like:

Pass Percentage Avg Per Week = averagex(values('Date'[WeekOf]),[Pass Percentage]))

you can do that by median, min, max, etc.

Respectfully,
Zoe Douglas (DataZoe)

See my reports and blog at https://www.datazoepowerbi.com/

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