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FAQs

What are reasonable limits and how are they used in WinSPC?

Easy to understand, reasonable limits offer remedies to a variety of scenarios that are occasionally problematic--in particular, two scenarios relating to data collection and one related to histograms.

Understanding Reasonable Limits

Reasonable limits are values that mark the boundary of reasonable data. Values that fall outside of reasonable limits are considered to be erroneous, or unreasonable. They are commonly attributable to data-entry errors--pressing '9' on a keyboard instead of '0', for instance. Reasonable limits are always wider than specification limits and control limits. In WinSPC, reasonable limits are primarily used to:

  • Detect and reject unreasonable data
  • Route ambiguous data from a device
  • Manipulate the amount of blank space on a histogram

Using Reasonable Limits to Detect and Reject Unreasonable Values

To preclude the complications and false alarms that unreasonable values can prompt, many operations automatically test incoming data against reasonable limits and block values found to be unreasonable from entering the WinSPC database. Following is one procedure to do this in WinSPC:

  1. In the Administrator window, double-click the collection plan that contains or will contain the collection plan step for which you would like values tested against reasonable limits.
  2. Create the desired collection plan step, if it hasn't already been created.
  3. With the desired step selected, click the Characteristic Tests button.

    characteristic_tests_button.png

  4. In the Test Selection dialog that is displayed, under Sample Level Tests, select Out of reasonable limits if it doesn't already have a green check beside it. (If it has a green check beside it, the test is already enabled. In this case go to step 6 below.)

    test_selection_1.png

  5. Click the Enable Test button at the top of the prompt.

    enable_test_1.png

  6. With the test selected, click the Add Trigger button in the top right of the prompt.

    add_trigger_button.png

  7. In the Trigger Setup prompt that appears, at Trigger Type, select Reject.

    trigger_setup_1_copy_copy_copy.png

  8. Click the Trigger Setup prompt's OK button.
  9. Click the Test Selection prompt's OK button.
Once the collection plan step is set up in this manner, any value that exceeds the reasonable limits will be rejected. If desired, additional triggers can be associated with the Out-of-reasonable limits test to cause other events, for example notifying users of the rejection, to automatically occur.

Using Reasonable Limits to Route Ambiguous Data from a Device

Sometimes, when collecting data from an external data source, the collection plan step to which a reading should be routed is ambiguous. Under certain circumstances, this ambiguity can be resolved with reasonable limits.

For a full explanation of using reasonable limits to route ambiguitous data from a device , see Matching external data source readings with collection plan steps in our Tips & Tricks section.

Using Reasonable Limits to Manipulate the Amount of Blank Space on a Histogram

In WinSPC, the visible area of a histogram is bounded by the lower reasonable limit on the left and the upper reasonable limit on the right. In some cases, this results in too much or too little blank space preceeding the first histogram bar and following the last histogram bar. To increase this blank space, the histogram variable's reasonable limits can be widened. Similarly, to decrease this blank space, the histogram variable's reasonable limits can be tightened.

Specifying Reasonable Limits

Only variables have reasonable limits. The concept doesn't make sense for attributes. For instructions on specifying a variable's reasonable limits, see Setting Up WinSPC > Setting Up Parts or Processes > Setting Up Variables > Setting Up Variable Specifications (Advanced) in WinSPC's Help system.
 
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