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7.
Product and Process Comparisons
7.1. Introduction
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| Detecting trends by plotting the data points to see if a line with an obviously non-zero slope fits the points | Detecting trends is equivalent
to comparing the process values to what we would expect a series of numbers to look like if there were no
trends. If we see a significant departure from a model where the next observation is equally likely to go up or down, then we would reject the hypothesis of "no trend".
A common way of investigating for trends is to fit a straight line
to the data and observe the line's direction (or slope). If the line looks horizontal,
then there is no evidence of a trend; otherwise there is. Formally, this is done by testing
whether the slope of the line is significantly different from zero. The
methodology for this is covered in Chapter
4.
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| Other trend tests | A non parametric approach for detecting significant trends known as the Reverse Arrangement Test is described in Chapter 8.Ê | ||
