qlik-control-charts

qlik-control-charts

Materials for and from my presentation to the Qlik Healthcare Dev Group September Gathering 2017-09-14.

  • See attached powerpoint for examples of these charts
  • Most charts are built in a similar manner - but the method of calculating SD varies for each one
  • Don’t use the build in standard deviation function

P Chart

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In QlikView your final chart properties will look something like this.
These chart expressions also work in Sense.

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Assuming you are using the sample data, we will need to specify the ‘Type’ variable in the set analysis. This may not be necessary with your real life data

p

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//p

Sum ({$<Type={“P”}>}Numerator/Denominator)

Use a slider (available within Sense as part of the Qlik dashboard extensions, or within QlikView), define a variable vNumCLPoints, and assign it to the slider.

For SPC, typically between 11 and 25 data points forms a good basis for initial control limits

Centre Line

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=// Centre line
if(RowNo()=1,RangeAvg(below(p,0,$(vNumCLPoints))), If(RowNo()> 1, Above((Centre))))

This expression starts at row 1 and calculates the average of the first n rows - where n is the number of points returned by the slider you have set up above. From row 2 onwards it refers back to the row imediately above.
This is very much like typing a formula in cell A1 in Excel, then typing =A1 in cell A2, before dragging the formula downwards. In short - we are locking the centre line to the average of these first n rows.

Now the key to getting this working properly - calculating the standard deviation.

SD

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=//SD

sqrt((Centre * (1 - Centre))/ sum({$<Type ={“P”}>}[Denominator]))

Control Limits

There are Upper and Lower Control limits, and Upper and Lower Warning Limits.

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// UCL Centre + (3*SD)

//LCL If(Centre - (3SD) < 0,0,Centre - (3SD)) // if LCL < 0, then 0, else LCL

=//UWL Centre + (2 * SD)

// LWL If (Centre - (2SD) < 0, 0, Centre - (2SD))

For the lower control and warning limits, if the value is less than 0, then we plot 0, else the calculated limits

Optional - highlight points outside limits

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This is optional. You might like to style the control limits to denote the control period by alternating between solid ( control period) and dashed (outside control period)

You may also want to highlight points outside the limits.

The following expressions should help -

//Chart Line Style Attribute if(RowNo() <= vNumCLPoints, ‘', '')

Repeat for each control limit as required

=//if outside the limits, display the value, else 0

IF(p > UCL,p, IF((p < LCL),p, IF((p > UWL AND p < UCL),p, IF((p < LWL AND p > LCL),p, 0 ))))

colour points outside limits

// beyond limit
IF(p > UCL,RGB(255,0,0), IF((p < LCL),RGB(255,0,0), IF((p > UWL AND p < UCL),RGB(255,128,0), IF((p < LWL AND p > LCL),RGB(255,128,0), RGB(141,170,203) ))))


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