Help Manual

Contents






Sigma Magic Help Version 15

Lot Acceptance Sampling Plan

Overview

Lot Acceptance Sampling Plans can be used to determine the strategy to accept a lot based on a sampling plan. This analysis is used especially when it is very expensive or time-consuming to inspect every incoming part from a supplier to determine if it is acceptable or not. The acceptance plan will indicate based on your inputs how many samples to inspect the lot and when to accept the lot and when to reject the entire lot.

This tool can be added to your active workbook by clicking on Stats and then selecting Sample Size > Lot Acceptance Plan.

Inputs

Click on Analysis Setup to open the menu options for this tool.

Setup

A sample screenshot of the setup menu is shown below.
inputs
1
Measurement: Specify the type of measurement you are working with. The available options are:
OptionDescription
DefectsDefects are any item or service that exhibits a departure from specifications. A defect does not necessarily mean that the product or service cannot be used.
DefectivesDefectives refer to the entire product or service and refer to the condition that the product or service is not usable. A product may have many defects - not all of these defects may cause the product to be defective.
2
Lot Size: Specify the Lot Size. This is the number of units that need to be inspected. If you leave this field blank then it is assumed that the lot size is very large. You can also write INF for infinite lot size.
3
AQL: Specify the Acceptable Quality Level (AQL). This is the level of defects in your entire sample that would be acceptable to you. You would not complain to the supplier if the defects in the entire lot stay below this number! For example, if you say 1% AQL, this means that you accept that the entire lot can have 1% defects.
4
RQL: Specify the Reject Quality Level (RQL). This is the maximum defects you are willing to accept in the entire lot. Any level of defects would be unacceptable to you and you would complain to the supplier if the values exceed this number. For example, if you say 5% RQL, then any defects more than 5% would be totally unacceptable to you in the entire lot.
5
Confidence Level: Enter the confidence level (default 95%) for your analysis. This value is used to determine your type I errors (what is the probability of rejecting a good lot).
6
Power: Enter the power value (default 90%) for your analysis. This value is used to determine your type II error (what is the probability of accepting a bad lot).
7
Help Button: Click on this button to open the help file for this topic.
8
Cancel Button: Click on this button to cancel all changes to the settings and exit this dialog box.
9
OK Button: Click on this button to save all changes and compute the outputs for this analysis.

Charts

If you click on the Charts button, you will see the following dialog box. Charts
1
Title: The system will automatically pick a title for your chart. However, if you would like to override that with your own title you can specify a title for your chart here. Note that this input is optional.
2
Sub Title: The system will automatically pick a subtitle for your chart. However, if you would like to override that with your own subtitle you can specify a subtitle for your chart here. Note that this input is optional.
3
X Label: The system will automatically pick a label for the x-axis. However, if you would like to override that with your own label for the x-axis you can specify a different label here. Note that this input is optional.
4
Y Label: The system will automatically pick a label for the y-axis. However, if you would like to override that with your own label for the y-axis you can specify a different label here. Note that this input is optional.
5
X Axis: The system will automatically pick a scale for the x-axis. However, if you would like to override that with your values for the x-axis, you can specify them here. The format for this input is to specify the minimum, increment, and maximum values separated by a semi-colon. For example, if you specify 10;20 then the minimum x-axis scale is set at 10 and the maximum x-axis scale is set at 20. If you specify 10;2;20, then, in addition to minimum and maximum values, the x-axis increment is set at 2. Note that this input is currently disabled and you will not be able to change this setting.
6
Y Axis: The system will automatically pick a scale for the y-axis. However, if you would like to override that with your values for the y-axis, you can specify them here. The format for this input is to specify the minimum, increment, and maximum values separated by a semi-colon. For example, if you specify 10;20 then the minimum y-axis scale is set at 10 and the maximum y-axis scale is set at 20. If you specify 10;2;20, then, in addition to minimum and maximum values, the y-axis increment is set at 2. Note that this input is optional.
7
Horizontal Lines: If you want to add a few extra horizontal reference lines on top of your chart you can specify the values here. The format for this input is numeric values separated by semi-colon. For example, if you specify 12;15 then two horizontal lines are plotted at Y = 12 and Y = 15 respectively. Note that this input is optional.
8
Vertical Lines: If you want to add a few extra vertical reference lines on top of your chart you can specify the values here. The format for this input is numeric values separated by semi-colon. For example, if you specify 2;5 then two vertical lines are plotted at X = 2 and X = 5 respectively. Note that this input is optional.

Verify

If you click on the Verify button, the software will perform some checks on the data you have entered. A sample screenshot of the dialog box is shown in the figure below. Verify The objective of this analysis as well as any checks that are performed is listed in this dialog box. For example, the software may check if you have correctly specified the input options and entered the required data on the worksheet. The results of the analysis checks are listed on the right. If the checks are passed, then they are shown as a green-colored checkmark. If the verification checks fail, then they are shown as a red-colored cross. If the verification checks result in a warning, they are shown in the orange color exclamation mark and finally, any checks that are required to be performed by the user are shown as blue info icons.

Outputs

Click on Compute Outputs to update the output calculations. A sample screenshot of the worksheet is shown below. outputs The analysis reports the derived sampling plan. How many items to inspect in the lot and what is the acceptance number. If the number of defects in the inspected lot exceeds the acceptance number, you would reject the entire lot. Note that it is assumed that you select the lot that needs to be inspected at random. If you have specified a lot size, then the software will also report the zero-based sampling plan. These are the number of units you randomly inspect and you reject the lot even if you find one defective item.

The graph shows the acceptance probability depending on the number of defects present in the lot. This graph can be used to determine what is the impact of various values on defect rates on your probability of acceptance of the lot. You would want a steep operating characteristic curve so that if the defects in the lot are higher the probability of acceptance drops rapidly.

Notes

Here are a few pointers regarding this analysis:
  • There are two plans shown, select the plan based on your requirements and agreements with your suppliers.

Examples

Following examples can be found in the Examples folder.
  • Determine the acceptance sampling plan for an AQL = 2% and a RQL = 4% with a confidence of 95% and a power of 90% (Acceptance Sampling 1.xlsm).