Control Chart

    What is Control Chart?

    The control chart is a statistical analysis used to study how a process changes over time. Data are plotted in time order. Control charts are used to routinely monitor quality or simply you can say, control chart is process improvement technique.


    What is a process?

    A process is simply what we do. It can be filling out an expense report, checking a person into a hospital, burning a CD with music, driving to work, filling a prescription, etc. All these process generates the data. We take these generated data and plot on control chart to improve efficiency.

    There are many types of control chart, but we are going to use the basic control chart with Std dev method to understand the concept.

    Followings are some scenario where we can use the control chart

    1) In manufacturing to test if machinery are producing products within specifications.

    2) suppose you want to reduce the time it takes to admit someone to the hospital.

    3) If you want to know how views are going up or down for particular website.

    Components of Control Chat

    1) Avg line

    2) UCL (Upper Control line) = Avg Value + Std Deviation* Std Dev Multiplier

    3) LCL (Lower control line) = Avg Value - Std Deviation* Std Dev Multiplier

    So, basically we can see which are the points "Out of Control" or "In Control".

    You can control your Std Dev multiplier using variable.

    How to create the Control Chart.

    1) Create the Combo Chart.

    2) Take any Time dimension of your interest which is suited for your analysis, here I have taken the MonthYear. Also    make it Continuous in Axis setting

    3) Create two Expression, one for Line and second for Symbol.

    4) Create expressions for Avg line, UCL & LCL which will be used as reference lines in the chart. Check the Chart properties ->Presentation for reference line setting.

    5) Create the Background color setting for Symbol expression

    if( sum(Sales) < vUCL and sum(Sales) > vLCL, LightGreen(), LightRed())


    6) Finally the chart will look like below



    There are some other useful links available to create the control charts

    QlikView Control (SPC) Chart


    See also :

    Benford's law

    Bollinger Bands