How to calculate the Lot Reject Rate

H

Helenliu

#1
I go straight to the topic, Pls help to review and give some suggestion.Thanks a lot.

There are several steps for manufacturing the product, and there is sampling inspection for each lot each step, and calculate lot reject rate by this farmula: reject lot number in the month/ total inspection lot in the month. And if they find one reject unit in the lot, operator will rescreen the whole lot, but for the lot, we count it as reject.
For example:
Step 1: 2 (reject lots in the month)/ 100(total lot qty sampled in the month)=2.00%
Step 2: 3 (reject lots)/98=2.04%
.
.
.
.
Step 10: 1(reject lot)/101=0.9%

My question is how to calculate the total reject ratio of the whole process.
 
Elsmar Forum Sponsor
S

Surehawk

#2
How can you have 101 lots in the last process when you only had 100 in the first (and rejected 2 of them)? Is the lot size different between the steps in the process?

If the lots were the same through the process it would just be to add together the failed lots and divide with the total number of lots manufactored in step 1.

Otherwise you will have to recalculate after the lots in each step. For example

Step 1. 100 parts per lot (set 100 as standard) 2 (rejected) X 1 (indexnumber) =1
Step 2. 2 (rejected) X 1 (since it is same lotsize) = 1
.
.
.
Step. 10 75 part per lot 1 (reject) X 0,75 (lower index since less in lots) = 0,75

And then add the rejected lots and parts of lots together and divide with total lots made in process 1.

Or else maybe I missunderstood the whole question and I am making a fool of myself.
 
#3
I go straight to the topic, Pls help to review and give some suggestion.Thanks a lot.

There are several steps for manufacturing the product, and there is sampling inspection for each lot each step, and calculate lot reject rate by this farmula: reject lot number in the month/ total inspection lot in the month. And if they find one reject unit in the lot, operator will rescreen the whole lot, but for the lot, we count it as reject.
For example:
Step 1: 2 (reject lots in the month)/ 100(total lot qty sampled in the month)=2.00%
Step 2: 3 (reject lots)/98=2.04%
.
.
.
.
Step 10: 1(reject lot)/101=0.9%

My question is how to calculate the total reject ratio of the whole process.
Refer an existing thread - Lot Reject Rate - The use of ppm vs. % (per cent)

.
 

Bev D

Heretical Statistician
Leader
Super Moderator
#4
Re: How to calculate the Lot Reject Rate ?

You may wan tot think about "rolled throughput yield". this is the 'opposite' of the lot reject rate you are asking for, but it has the same intent and the math is easy.

since manufacturing is usually a 'continuous' process (new plots introduced before previous lots are through the process) there will almost always be a different number of lots that make it through any given process step in any given reporting period. this calcaultion takes care of that.

simply take the YIELD (the number of lots that passed the step divided by the total numbe rof lots completed [passed + rejected] at the step) of each step and multiply the proportions for each step together.

so if you had:
step 1 - 30 lots passed, 15 rejected, yield = 30/45 = .67
step 2 - 35 lots passed, 12 rejected, yield = 35/47 = .74
step 3 - 28 lots passed, 10 rejected, yield = 28/38 = .74

and RTY = .67*.74*.74 = .37 or 37%
 
H

Helenliu

#5
Re: How to calculate the Lot Reject Rate ?

Yes, it is rooling lot reject ratio. But it is only sampling inspection, maybe not lot based, but time base. let's make the lot as a unit.
If I samplied 5 lots in total 100 this month, I found one lot was reject. Then the LRR ratio=1/5*100% or =1/100*100%?
 
Last edited by a moderator:
Thread starter Similar threads Forum Replies Date
D How to calculate the A4 constants in Median/Range control charts? Statistical Analysis Tools, Techniques and SPC 2
S How to calculate Effective Number of People for ISO 13485 Certification? General Auditing Discussions 2
A Need to calculate tolerance Intervals with a set of non-normal data and 3-Parameter Weibull distribution Using Minitab Software 0
U Calculate acceptance of equipment reading after calibration General Measurement Device and Calibration Topics 2
U Calculate acceptance of calibration reading Measurement Uncertainty (MU) 1
S How would you calculate this? Capability, Accuracy and Stability - Processes, Machines, etc. 4
S Bend allowance formula to calculate per degree per radii Manufacturing and Related Processes 2
P Commercial excel templates that calculate overhead, hourly rates, fee proposal Excel .xls Spreadsheet Templates and Tools 0
M How to calculate benefits? Moving some developed non controlled software/automation systems Service Industry Specific Topics 2
qualprod Calculate number of people in processes, considering cycle time/takt time = quantity of people Lean in Manufacturing and Service Industries 9
S How to calculate the maximum allowable hysteresis of a pressure gauge General Measurement Device and Calibration Topics 1
S How to Calculate the producer Risk AQL - Acceptable Quality Level 25
qualprod Methods to Calculate Available Production Rate ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 0
K Why calculate LCL in P-chart? Statistical Analysis Tools, Techniques and SPC 4
B How to calculate CPK with n = 1 Manufacturing and Related Processes 2
A Add-On software for VMC to calculate setting time, Idle Time, OEE etc.. Manufacturing and Related Processes 2
L How to calculate the Probability of Deficiency of the AQL of an Inspection AQL - Acceptable Quality Level 16
V How to Calculate Unbiasing Constant C4 Statistical Analysis Tools, Techniques and SPC 1
J How to Calculate OEE Indicator for a Workshop (15 Machines) Lean in Manufacturing and Service Industries 6
R How to calculate UCL (Upper COntrol Limit) and LCL (Lower Control Limit) Statistical Analysis Tools, Techniques and SPC 5
T I want to calculate reliability at 750 hours Reliability Analysis - Predictions, Testing and Standards 8
R How To Calculate & Print CDF and Lower Bound CDF value for some t(time) with Minitab? Using Minitab Software 1
N How to calculate the Premium Freight charges Service Industry Specific Topics 4
A How to calculate Mean and standard deviation without sample size in Minitab Using Minitab Software 5
R When do I need to calculate Process OEE? Manufacturing and Related Processes 12
kedarg6500 How to calculate Thread Pitch Diameter for non standard TPI like 5/8" 17.75 NF3 Manufacturing and Related Processes 3
C Definition and Way to Calculate some Metrics Lean in Manufacturing and Service Industries 6
M How calculate P value for Linearity and Bias using Excel Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 8
N Formula that will calculate the NoGo Major Diameter of an M20 x 1.5 6H Thread gage Inspection, Prints (Drawings), Testing, Sampling and Related Topics 3
M How to calculate Cg and Cgk for a 2-Dimensional Area Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 4
T Calculation Question - Calculate the runner weight for a part Manufacturing and Related Processes 2
M Spreadsheet on How to Calculate SMT Capacity wanted Manufacturing and Related Processes 2
F How to Calculate the unbiased Standard Deviation in Excel using Minitab Formula Capability, Accuracy and Stability - Processes, Machines, etc. 6
S Spreadsheet that will Calculate Audit Duration times based on IAF MD5 and MD11 Excel .xls Spreadsheet Templates and Tools 8
M Calculate PPMs by going back to Date of Production or Date Noticed Quality Tools, Improvement and Analysis 7
M How to calculate MTTF (Mean Time to Failure) Reliability Analysis - Predictions, Testing and Standards 1
L How calculate the number of samples Statistical Analysis Tools, Techniques and SPC 2
A Simple 125 piece part Capabilty Excel .xls sheet to calculate Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 3
R How to calculate MMC when is placed after datum Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
S Which one is better to calculate, Cpk or Ppk? Statistical Analysis Tools, Techniques and SPC 11
C How to calculate P value on Probability Plot Statistical Analysis Tools, Techniques and SPC 1
S Supplier Disruption Score and how to calculate it Quality Tools, Improvement and Analysis 4
D IAQG OASIS - New tool to calculate Audit Time AS9100, IAQG, NADCAP and Aerospace related Standards and Requirements 1
M How to calculate dimensions of trapezoid ? Quality Tools, Improvement and Analysis 3
J How to calculate Surveillance Audit Days in Corporate Audit Scheme IATF 16949 - Automotive Quality Systems Standard 4
S How to calculate Supplier PPM at Receiving Inspection Document Control Systems, Procedures, Forms and Templates 2
K How to Calculate Sample Size at 95% Confidence Interval on Surface Roughness Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
R How to calculate the value add time? Lean in Manufacturing and Service Industries 2
G Should I calculate both Cp and Cpk for a Unilateral Tolerance? Statistical Analysis Tools, Techniques and SPC 8
H Continuous Monitoring Instrument Data Validation and how to calculate Outliers Quality Tools, Improvement and Analysis 1

Similar threads

Top Bottom