SPC charts for time-based data?

[Former Member]

I'll try to be succinct in explaining what I'm trying to achieve.

We run batch processes, and at various points in the process we take samples and test them. The sample results should be increasing exponentially over the course of the batch. I have data from a lot of batches in x/y format - x is the duration of the batch and y is the sample result. It makes a very noisy but clearly exponentially increasing data set.

What I want is to calculate some control limits that will obviously have to be increasing over time as well. I want to be able to say that, for a current batch, a sample at X time does or does not fall within the expected historical range.

Is this even possible with xMII, either in the logic editor or the iSPC charts?

I'd like to make the same sort of chart for other time-based samples that have more complex trends over the duration of a batch, not simply exponential or linear.

[Former Member]

Hi, Carrie.

Your application doesn't really sound like what I would call an "SPC" chart, but rather, more of a so-called "golden batch" chart. This type of visualization is used when a production activity should have an expected set of value(s) and/or a shape to a curve. You would then "overlay" actual values onto this curve to track actual vs. expected and perhaps to highlight deviations.

This can best be done by combining a multiple queries using BLS (one or more queries for the "limits" and expected values, one query for the actual values), and combining/merging them so that the charting engine (either a line/marker chart or an XY plot) sees them as coming from a single source.

With some creativity, you can even have the "outlier" points displayed in another color. The xMII application team can help you with setting this up - Jeremy may still have a chart template/transaction I put together a while back called "LimitChart", which could be used for your application.

Rick

[Former Member]

Rick,

Thanks for your response. I was hoping to take advantage of the inherent SPC calculations rather then building them myself in the BLS. I'd be very interested in seeing the LimitChart templates and transactions.

Thanks,

Carrie

[Former Member]

SPC isn't really appropriate for changing limits such as you have in your application. If you created some type of "normalization" model, whereby you divided the actual by the ideal value (so that from an SPC point of view the target value was always 1.0), you might be able to make it work to detect drifts and out of control situations.