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How to measure reliability

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Abusing the word "Reliability" was an annoying thing for me, it's not linked to submission date of a document nor the training programs, yes these procedure can help in undirect way to improve the reliability, but when you consider your reliability program sole on it, then you are not doing reliability anymore.
So i decided to express my anger in peaceful way and i hope it can be a postive too.
for that i'll start to write a post and i'll call it "Real Reliability" to bust the myth around reliability, and i'll start with my first enemy "MTBF".
This for all the fed up guys from the wrong usage of "Reliability"

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How to measure reliability

  1. 1. Ammar Alkhaldi Real Reliability 15 March 2016 PART 1: why you shall not use MTBF!!! How to Measure Reliability
  2. 2. Measurement can help us to answer the followings question: Are we doing good or bad ? Is our performance increasing or decreasing ? Which unit is performing better ? (Benchmarking) What/How to improve ? “You can’t improve what you can’t measure” Why are we measuring things ?
  3. 3. 1. MTBF is a misleading indicator.  Example: 1000 Units, one unit fail @ 1 Hour, MTBF = 1000 Hours 1 Unit fail @ 1000 hours, MTBF = 1000 Hours Is it the same ?  First of all:- How you shall not measure reliability!!! 𝑴𝑻𝑩𝑭 = 𝜽 = 𝑶𝒑𝒆𝒓𝒂𝒕𝒊𝒏𝒈 𝑯𝒐𝒖𝒓𝒔 # 𝑭𝒂𝒊𝒍𝒖𝒓𝒆𝒔 How to measure reliability ?
  4. 4. 2. MTBF Can’t be used for benchmarking.  Example: SYSTEM #2 seems to be performing better  First of all:- How you shall not measure reliability!!! 𝑴𝑻𝑩𝑭 = 𝜽 = 𝑶𝒑𝒆𝒓𝒂𝒕𝒊𝒏𝒈 𝑯𝒐𝒖𝒓𝒔 # 𝑭𝒂𝒊𝒍𝒖𝒓𝒆𝒔 How to measure reliability ? SYSTEM January February SYSTEM #1 150 𝐻𝑜𝑢𝑟𝑠 6 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 25 690 𝐻𝑜𝑢𝑟𝑠 15 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 46 SYSTEM #2 540 𝐻𝑜𝑢𝑟𝑠 18 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 30 300 𝐻𝑜𝑢𝑟𝑠 6 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 50
  5. 5. 2. MTBF Can’t be used for benchmarking.  Example: But not really. Any sense ?  First of all:- How you shall not measure reliability!!! 𝑴𝑻𝑩𝑭 = 𝜽 = 𝑶𝒑𝒆𝒓𝒂𝒕𝒊𝒏𝒈 𝑯𝒐𝒖𝒓𝒔 # 𝑭𝒂𝒊𝒍𝒖𝒓𝒆𝒔 How to measure reliability ? SYSTEM January February TOTAL SYSTEM #1 150 𝐻𝑜𝑢𝑟𝑠 6 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 25 690 𝐻𝑜𝑢𝑟𝑠 15 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 46 840 𝐻𝑜𝑢𝑟𝑠 21 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 40 SYSTEM #2 540 𝐻𝑜𝑢𝑟𝑠 18 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 30 300 𝐻𝑜𝑢𝑟𝑠 6 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 50 840 𝐻𝑜𝑢𝑟𝑠 24 𝐹𝑎𝑖𝑙𝑢𝑟𝑒𝑠 MTBF = 35
  6. 6. 3. MTBF is time independent.  Example: 12 failures over 12 months, MTBF = (365/12) = 30.4 MTBF = 30.4  First of all:- How you shall not measure reliability!!! 𝑴𝑻𝑩𝑭 = 𝜽 = 𝑶𝒑𝒆𝒓𝒂𝒕𝒊𝒏𝒈 𝑯𝒐𝒖𝒓𝒔 # 𝑭𝒂𝒊𝒍𝒖𝒓𝒆𝒔 How to measure reliability ? M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 F1 F2 F3 F4 F5 F7F6 F8 F9 F10 F11 F12
  7. 7. 3. MTBF is time independent.  Example: 12 failures over 12 months, MTBF = (365/12) = 30.4 MTBF= 30.4, But the failure rate is increasing?  First of all:- How you shall not measure reliability!!! 𝑴𝑻𝑩𝑭 = 𝜽 = 𝑶𝒑𝒆𝒓𝒂𝒕𝒊𝒏𝒈 𝑯𝒐𝒖𝒓𝒔 # 𝑭𝒂𝒊𝒍𝒖𝒓𝒆𝒔 How to measure reliability ? M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 F 1 F 2 F 3 F 4 F 5 F 7 F 6 F 8 F 9 F 1 0 F 1 1 F 1 2
  8. 8. 3. MTBF is time independent.  Example: 12 failures over 12 months, MTBF = (365/12) = 30.4 MTBF= 30.4, But the failure rate is decreasing?  First of all:- How you shall not measure reliability!!! 𝑴𝑻𝑩𝑭 = 𝜽 = 𝑶𝒑𝒆𝒓𝒂𝒕𝒊𝒏𝒈 𝑯𝒐𝒖𝒓𝒔 # 𝑭𝒂𝒊𝒍𝒖𝒓𝒆𝒔 How to measure reliability ? M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 F 1 F 2 F 3 F 4 F 5 F 7 F 6 F 8 F 9 F 1 0 F 1 1 F 1 2
  9. 9. 3. MTBF is time independent.  Example: 12 failures over 12 months, MTBF = (365/12) = 30.4 MTBF= 30.4, But the failure rate is decreasing? When to plan PMs ?  First of all:- How you shall not measure reliability!!! 𝑴𝑻𝑩𝑭 = 𝜽 = 𝑶𝒑𝒆𝒓𝒂𝒕𝒊𝒏𝒈 𝑯𝒐𝒖𝒓𝒔 # 𝑭𝒂𝒊𝒍𝒖𝒓𝒆𝒔 How to measure reliability ? M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 F 1 F 2 F 3 F 4 F 5 F 7 F 6 F 8 F 9 F 1 0 F 1 1 F 1 2
  10. 10. 4. MTBF considering normal distribution, is your data so ?  Example: But first, how different distribution can make different result/decision ?  First of all:- How you shall not measure reliability!!! 𝑴𝑻𝑩𝑭 = 𝜽 = 𝑶𝒑𝒆𝒓𝒂𝒕𝒊𝒏𝒈 𝑯𝒐𝒖𝒓𝒔 # 𝑭𝒂𝒊𝒍𝒖𝒓𝒆𝒔 How to measure reliability ?
  11. 11. Here is the beautiful normal distribution AKA Bell shape. Where MEAN = MEDIAN = MODE The normal distribution MEAN TIME BETWEEN FAILURES So we are talking about the mean, and our X-axis is time, and Y-Axis is failures
  12. 12. Here is the beautiful normal distribution AKA Bell shape. Where MEAN = MEDIAN = MODE IS YOUR DATA FOLLWING THE NORMAL DISTRBUTION ? The normal distribution MEAN TIME BETWEEN FAILURES So we are talking about the mean, and our X-axis is time, and Y-Axis is failures
  13. 13. Here is the beautiful normal distribution AKA Bell shape. Where MEAN = MEDIAN = MODE IS YOUR DATA FOLLWING THE NORMAL DISTRBUTION ? Let’s see The normal distribution MEAN TIME BETWEEN FAILURES So we are talking about the mean, and our X-axis is time, and Y-Axis is failures
  14. 14. Let’s say we are studding the failure of lightbulb, we have a group of 100 bulb, and we are running in the constant failure rate part of the bath curve (Phase 2) The normal distribution
  15. 15. Let’s say we are studding the failure of lightbulb, we have a group of 100 bulb, and we are running in the constant failure rate part of the bath curve (Phase 2), we’ll assume this rate = 1%, Remember MTBF = 1/failure rate MTBF = 1/1% = 100 MTBF = 100 The normal distribution
  16. 16. Let’s say we are studding the failure of lightbulb, we have a group of 100 bulb, and we are running in the constant failure rate part of the bath curve (Phase 2), we’ll assume this rate = 1%, Remember MTBF = 1/failure rate MTBF = 1/1% = 100 MTBF = 100 So half of the population should be failed by the @ 100 hours Let’s try it The normal distribution
  17. 17. The data points will followings:- 100 – 1% = 99 99 – 1% = 98.01 98.01-1%= 97.02 97.02 – 1% = 96.05 And so on… @ 100 hours we left with 37 units… But why ? We suppose to get MEAN=50 unit ??? Simply because the failure pattern unfirming an exponential distribution. For exponential : MEAN ≠ MEDIAN ≠ MODE But is everything followings exponential pattern ? NO EVERY FAILURE MODE HAVE IT’S UNIQUE DISTRBUTION SHAPE. The normal distribution 0 20 40 60 80 100 120 1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253 265 Units Time in Hours @ 100 hours only 37 units survives
  18. 18. If you think MTBF is not the wright way to measure reliability then stay toned for the upcoming post. Salam Ammar Alkhaldi, CSSBB So how to measure reliability then ?

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