What is the different between a trend chart and a run chart?
Answer:
Different industries have different methods, often customized for what works best for their industry/situation, but here is how it worked when I used to deal with SPC.
A run chart shows a parameter that you're measuring as a function of time (run order). A trend chart has time as a factor (run order), but is often sorted by another factor, such as product type. To further confuse things, sometimes a run chart can BE a trend chart as well, if it is running one process or product for a while.
Example: Lets say you have three tools running three different processes, let's say three lathes running aluminum, steel, and bronze rods. Any material could be run on any lathe at any time, based on whatever it was decided to run. For simplicity, let's say all three lathes intend to turn out rods that measure 1.0000" in each material. The run chart would report the thickness of each rod that came off a machine in the order they were run regardless of material. A trend chart would tend to be sorted by something else, say material. You could also make a trend chart that would show a specific material from a specific machine.
Each chart allows you to look at the data from a different perspective, and draw conclusions about what needs to be done to achieve optimal quality (hitting the target thickness) and reduce scrap. Let's say a run chart shows that tool A is producing thicker and thicker product. This could mean that you need to change the carbide tip which is getting worn and/or move the carbide tool closer to the rod to reduce the thickness of the rods, but other things could be going on. You notice that tool A has run a lot of aluminum rods recently. You pull up a trend chart of all aluminum rods run recently and you notice that aluminum rods are running unusually thick. You run a trend chart for each individual tool for aluminum, and notice that aluminum is consistantly thick on every tool. Apparently you need to adjust ALL tools in when running aluminum rods.
In the above example, what at first looked to be a tool problem, instead turned out to be a process problem with aluminum. Further research on the data could be done to find if this is caused by a specific batch of incomming material, or if this is a problem which has been ongoing.
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