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Z-charts

Quality Tools > Tools of the Trade > 18: Z-charts

 

One of the problems with charting information is where your objective is to explain a complex situation with a minimum number of charts or graphs. The Z-chart is a combination chart that shows three perspectives in a single picture.

 

How does it work

Showing progress over time of a small project or a whole business can result in many different charts to show various viewpoints. Z-chart can reduce at least three different charts into one simple chart.

 

When reviewing progress of a project or business, there are three timescales you can look at.

       In the short term, you will want to know 'How did we do this month (/week/etc.)?'.

       In the longer term, the rate of growth or decline may be of significant interest.

       In the intermediate term, it is interesting to connect these two, to see how short-term achievement is building up to longer-term goals.

 

The Z-Chart below thus shows you things like:

       Over the past 12 months, sales have been up and down, ranging from 2M to 12M per month. Across the months, there has been no visible trend.

       Looking at the longer-term rolling 12 month figures, there is what seems to be a visible upward trend.

       The medium-term line shows how sales have built up over the past 12 months.

 

 

Z-Chart


Z-chart: How to do it

1. Gather data on a regular basis for a given activity. This may be the sales per month, words written by an author per week or calls handled by a customer response centre per day.

 

2. Identify the major review period to be considered. When you are gathering data on a monthly cycle, then this is likely to be 12 months. If your minor review period is one day, then the major review period is more likely to be something like one month or four weeks.

 

2. For each minor review period (one month in the examples here), build the longer-term view by totalling the data for the past major review period (12 months in the examples here).

 

Note that the rolling total for 12 months ago will require data from 12 months previous to that. Thus the longer-term view really does look back over a longer period.

 

3. For the current review (month, in the example here), sum the previous values over the past major review period (12 months here).

 

4. Build the Z-chart, as in the diagram above.

 

The table below shows how these measurements can all be put into a simple table. You can use a spreadsheet to automatically build the third and fourth column and also to subsequently build the final Z-chart.

 

 

This month

Total since March

Rolling 12 month total

March

5

5

46

April

2

7

45

May

5

12

47

June

12

22

46

July

3

25

48

August

5

30

47

September

7

37

49

October

2

39

50

November

4

43

51

December

2

45

50

January

7

52

58

February

3

55

55

 

 

Next time: IDEF0

 

This article first appeared in Quality World, the journal of the Institute for Quality Assurance

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