Help Manual

Contents






Sigma Magic Help Version 18

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 mainly used when it is very expensive or time-consuming to inspect every incoming part from a supplier to determine whether it is acceptable. Based on your inputs, the acceptance plan will indicate how many samples to inspect the lot, 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 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, the lot size is assumed to be 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, you accept that the entire lot can have 1% defects.
4
RQL: Specify the Reject Quality Level (RQL). You will accept This maximum number of defects in the entire lot. Any 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 more than 5% defects would be 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 (the probability of rejecting a good lot).
6
Power: Enter the power value (default 90%) for your analysis. This value determines your type II error (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

You will see the following dialog box if you click the Charts button. Charts
1
Title: The system will automatically pick a title for your chart. However, if you want to override that with your 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 want to override that with your subtitle, 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 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 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 determine the minimum, increment, and maximum values separated by a semi-colon. For example, if you specify 10;20, 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 determine the minimum, increment, and maximum values separated by a semi-colon. For example, if you specify 10;20, the minimum y-axis scale is set at 10, and the maximum y-axis 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, two horizontal lines are plotted at Y = 12 and Y = 15, respectively. Note that this input is optional.
8
Vertical Lines: You can specify the values here if you want to add a few extra vertical reference lines on top of your chart. The format for this input is numeric values separated by semi-colon. For example, if you specify 2;5, two vertical lines are plotted at X = 2 and X = 5, respectively. Note that this input is optional.

Verify

If you click the Verify button, the software will perform some checks on the data you entered. A sample screenshot of the dialog box is shown in the figure below. Verify The software checks 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, they are shown as green-colored checkmarks. If the verification checks fail, they are shown as a red-colored cross. If the verification checks result in a warning, they are shown in the orange exclamation mark, and finally, any checks that are required to be performed by the user are shown as blue info icons.
1
Item: The left-hand side shows the major tabs and the items checked within each section
2
Status: The right-hand side shows the status of the checks.
3
Overall Status: The overall status of all the checks for the given analysis is shown here. The overall status check shows a green thumps-up sign if everything is okay and a red thumps-down sign if any checks have not passed. Note that you cannot proceed with generating analysis results for some analyses if the overall status is not okay.

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 will be inspected in the lot, and what is the acceptance number? If the defects in the inspected lot exceed the acceptance number, you will reject the entire lot. Note that it is assumed that you select the lot that must be inspected randomly. If you specify a lot size, 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 in the lot. This graph can be used to determine the impact of various values on defect rates and the likelihood 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:
  • Two plans are shown; select the plan based on your requirements and agreements with your suppliers.

Examples

The following examples are in the Examples folder.
  • Determine the acceptance sampling plan for an AQL = 2% and an RQL = 4% with a confidence of 95% and a power of 90% (Acceptance Sampling 1.xlsx).



© Rapid Sigma Solutions LLP. All rights reserved.