Using SPC but with variable target/USLHi, I am looking for some advice using SPC to monitor Actual Yield vs Planned Yield.
Plotting the SPC to measure Actual Yield (%) alone is straightforward, and normally when there's a single fixed value for an USL or a Target, this is also easy to add. However, for each month there's a Planned Yield value and this varies from month to month. The Target or USL seem to require a fixed value - so what's the best way to represent Actual Yield vs the Planned Yield in an SPC chart?
If there's a better way than SPC, such as standard graphing, then also keen to hear. Appreciate this would be fairly easy to do in Excel if that's the recommended way.
GraphicsRe: Using SPC but with variable target/USL.Here's my thought: Create a variable called "Yield Residual" or something that is descriptive to your audience where the plotted variable is just the Actual - Planned value. You can use an individuals and moving range chart to monitor the stability of the residual value.
Re: Using SPC but with variable target/USLSorry for my possible misinterpretation of your question...Here are my thoughts:
1. SPC has nothing to do with target values or spec. limits. It was invented by Shewhart to understand consistency/stability of the within subgroup sources of variation and then, if appropriate, compare those sources to the between subgroup sources of variation to determine where to investigate.
“The engineer who is successful in dividing his data initially into rational subgroups based on rational theories is therefore inherently better off in the long run. . .” Shewhart, Economic Control of Quality of Manufactured Product
2. SPC is NOT a monitoring tool! It is used to answer specific questions about the process. This does not mean you can't plot the data in a time series, but that is not SPC. I believe you should understand the consistency and stability of actual yields (and if it were me I would want to understand what affects yield and how to reduce the variation in yields). To do this requires first a set of hypotheses regarding what factors affect the yield and then directed sampling to separate and assign the causal factors (or perhaps DOE to speed the investigation up). You should also seek to understand the variation in planned yields (is this just a negotiation or are there factors driving these changes?)
I recommend reading Don Wheeler's book: Understanding Statistical Process Control
Re: Using SPC but with variable target/USLHi there,
Yeah, this method had crossed my mind as a potential way to go forward. However it means that if people are still interested in the raw value then there's another graph needed. Thanks though!
