Creating and managing sampling plans

Sampling plans are used to calculate a sample size automatically, based on the quantity of items specified in an inspection order. They are templates that can be reused in different inspection orders where samples need to be taken according to identical rules. Sampling plans allow you to specify a series of consecutive quantity (From – To) ranges with the corresponding sample size to be taken from each. A field labelled Calculation method decides whether the specified sample size for a given range is interpreted as a fixed value or as a percentage.

To illustrate the use of sample size and calculation method, let us assume that, as a general rule, you inspect 10% of the units that come off a production line. However, to be more lean in your approach, you know you need to test less the more units you create, as the likelihood of failures is less when the machines do not need recalibration (which is needed when creating a different product). So, for example, if you create up to 100 items, 10% of that number will be tested. When you create more than 101 units, you can scale back your testing by say, 5%. Then, if you create over 201 units, you limit the number of samples taken to a total of 20. This way, you will ensure that operators do not test more than is needed to get a clear result of the inspection order.

In the following example, we have used the sampling plan structure described above:

Another typical case is when tests are done per batch. In this setting, you may want to create fixed plan for single samples, such as the one below.

Creating sampling plans

To create a new sampling plan, click the + Add new button in the top right corner of the Inspect / Manager screen, and select Sampling plan from the drop-down list. (Alternatively, you can hit Add sampling plan on the sampling plans list page.)

Provide a descriptive name for your new sampling plan in the Name field. and then set up the rules for sampling.

The rules determine the volume of the sample to be taken from the item.

You can specify any number of rules but creating at least one rule is mandatory.

A rule consists of three components: a From - To range, the Sample size and a Calculation method.

The Create new sampling plan page (shown in the screenshot above) allows you only to enter the first rule of your sampling plan only for which you can specify the Sample size and a Calculation method. The From - To range entry is not available for the first rule. Once you have saved your new sampling plan, you can return to it through either by selecting View sampling plan on the save bar or via the Sampling plans main menu entry and add range and further rule information.

Enter a numeric value in the Sample size field to specify the number (amount) of samples to be taken. Then select one of the two options offered by the Calculation method drop-down list.

The calculation method should be Fixed if you want to use the sample size value directly. In this case, the operator will take as many samples as the numeric value entered in the Sample size field. Select Percent as your calculation method to interpret the numeric value entered in the Sample size field as the percentage of the total quantity of items. In this case, a Sample size value of n means that the number of samples will be the n % of the total quantity of items.

You can enter any value in the Sample size field, thus a percentage of greater than 100% is also allowed. However, you should be aware that the number of samples created should not exceed 500. If the number of samples is greater than 500, then the resulting inspection order cannot be released.

Managing sampling plans

If you need to add further rules and/or range information after specifying the first rule as described above, or if you wish to edit the details of a sampling plan, then select the Sampling plans menu entry on the left. The list of available sampling plans will be displayed.

Press Add sampling plan in the top right corner to create a new plan. Clicking the options (°°°) icon on the right side of the list allows you to delete and edit the given sampling plan, but you can also edit a plan by simply clicking on its name. Editing will open the detail view of the sampling plan. By default, the General tab is displayed, where you can edit the name and description of the plan.

Open the Rules tab to add any number of new rule lines you need. Press + Add new sampling rule to insert a new rule. Enter the serial number of the first sample that you want to apply the new rule to in the From field. The To field on the previous line will automatically change to the serial number of the last sample in the previous rule. Enter the Sample size and select a Calculation method as described above.