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02-18-2012, 06:48 AM   #1
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How best to interpret SDM survey results?

Consider the result for the SDM reliability survey for the DA 16-50/F2.8 lens as of 18th Feb 2012:




Let's examine the 2009 figures:

Total OK Failed Failed %
75 41 34 45.3%
What is the failure rate?

Let's simplify. Consider only lenses purchased in 2009, looking at them up to 2011. Let's say these 2009 lenses were all bought at the beginning of that year.

Say we had a 20%/yr constant failure rate. Here is the result of 100 lenses purchased in 2009.

Year Start of Year OK at End of Year Failed Failed % of Original Amount
2009 100 80 2020%
2010806416 36%
2011 645113 49%
But lenses probably won't have a constant failure rate per year. They're probably like other mechanical devices: the longer you use it, the more likely it is to wear out. So let's consider years of use. Say it fails at a rate of 18.3%/years-of-use. So now the failure numbers look like:

Year Start of Year OK at End of Year Failed Failed %
2009 100 82 18 18%
201082 63 37 37%
2011 6351 49 49%


There are a number of other complications:

1. In reality, lenses were purchased throughout the year of 2009 so the avg. age of these lenses at the end of 2011 would be 2.5yrs.
2. Older purchasers are probably under-represented as 2009 purchasers are more likely then 2011 users to, by now, either have switched brands, or not use DSLRs at all, so they are less likely to come to this forum and participate in the survey
3. In this survey, more lenses are being purchased each year.
4. If we knew the year of failure of these 2009 purchases (2009, 20010, 2011), it would help to more accurately determine the failure rate and problem years of manufacture.

Here's what I've done. I've considered years-of-use. Since I don't know the year of failure, I'll assume they all failed today. I've started from 18th Feb 2012. This is the 49th day of 2012. The amount of years is 49/365/2 (divide here by 2 since some lenses were bought on 1st day of 2012 while others may have been bought on the 49th day, so the avg. is 24.5 days) = 0.07yrs.

Adding in 2011, we add 0.5 years (again since lenses could have been bought throughout that year) = 0.07 + 0.5 = 0.57yrs.

For 2010 and for earlier years we add the full year, so 2010 is 1.57yrs.

Here are the figures:

Year Elapsed Yrs OK Fails Total Fail % Fails-Yrs Total-Yrs Total Fail Rate %
2012 0.0714 4 18 22.2% 0.3 1.2 22.2%
2011 0.57 73 18 91 19.8% 10.5 52.8 19.8%
2010 1.57 55 23 78 29.5% 46.5 175.1 26.6%
2009 2.57 41 34 75 45.3% 133.8 367.6 36.4%
2008 3.57 30 29 59 49.2% 237.2 578 41.0%
2007 4.57 20 11 31 35.5% 287.5 719.6 39.9%
2006 5.57 2 0 2 0% 287.5 730.8 39.3%
2005 6.57 1 1 2 50% 294.1 743.9 39.5%


Here are 3 graphs:









The 60-250 curve is exponential as you'd expect for straight wear and tear failure. The failure rate is relatively low: only a total of 9% of lenses purchased within the last 2.5yrs have had SDM failures.

The other two lenses show high early failure rates and the rising part of the curve is not indicating a constant years-of-use failure figure.

Searching now for problem years of manufacture (YOM), let's look at the failure rate for years where the yearly total sample size is reasonably large (at least a total of 18 OKs + Fails in a year)

YOM Lens Fail %
2007 16-5035%
2008 49%
2009 45%
2010 29%
2011 20%
2012 22%
   
200750-135 34%
2008 42%
2009 30%
2010 21%
2011 7%
2012 24%
   
200960-25013%
2010 8%
2011 2%


Without knowing the year of failure, it is not possible to determine problem years of manufacture.

Dan.


Last edited by dosdan; 02-18-2012 at 01:16 PM.
02-18-2012, 08:45 AM   #2
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Now my head hurts.
02-18-2012, 08:46 AM   #3
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Lots of nice graphs and charts, but unregulated public polls are not reliable.
02-18-2012, 10:36 AM   #4
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QuoteOriginally posted by SpecialK Quote
Lots of nice graphs and charts, but unregulated public polls are not reliable.
Lots of nice lenses and optics, but SDM lenses are not reliable.


Last edited by builttospill; 02-18-2012 at 12:06 PM.
02-18-2012, 10:43 AM   #5
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QuoteOriginally posted by dosdan Quote
Without knowing the year of failure, it is not possible to determine problem years of manufacture.

Dan.
I don't follow your logic. Poll responders are asked year of purchase (which BTW may not be accurate for year of manufacture - depending on how long the lens was in stock), and if they had a failure. Statistically speaking, it doesn't matter if the failure occurs in the same year the lens was made or three years later, failure is failure. While I agree that a high rate of failure within the year of manufacture is certainly an indication of problems, it's no different than a similar rate over two years or three years - the failure rate remains the same. What I'm saying is if 100 lenses were produced in 2005 and 20 of them failed that same year, it's statistically no different than if only 20 failed over the course of the next 5 years - both represent a 20% failure rate.

In an ideal survey, we could track year of manufacture and compare it to usage, which would provide much more meaningful data, but alas, that's not going to happen.

Nice graphs BTW.
02-18-2012, 12:23 PM   #6
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The survey can't really determine a percentage of failures due to response bias, but it can say this much:
  • SDM fails on lenses that see extensive use, especially the 16-50, 60-250, and 17-70 zooms
  • The reliability of SDM lenses hasn't increased over time
  • The "budget" DC technology seems to be quite reliable, with no recorded failures

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02-18-2012, 01:27 PM   #7
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QuoteOriginally posted by Tom S. Quote
. What I'm saying is if 100 lenses were produced in 2005 and 20 of them failed that same year, it's statistically no different than if only 20 failed over the course of the next 5 years - both represent a 20% failure rate.
Statistically, no difference. But it would make a difference to the user experience. Say 20% of the 2009 date-of-purchase lenses failed quickly (within 0.5yrs) and then stopped failing, compared to a more spread-out failing (after an avg. of 2yrs). At least the spread-out failure rate would allow the avg. affected user more use of his/her lens before failure. (And less chance of getting it fixed under warranty!)

Dan.
02-18-2012, 06:20 PM   #8
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QuoteOriginally posted by builttospill Quote
Lots of nice lenses and optics, but SDM lenses are not reliable.
oh... what about the DA*55, DA*200 and DA*300 then?

QuoteOriginally posted by Adam Quote
The survey can't really determine a percentage of failures due to response bias, but it can say this much:
  • SDM fails on lenses that see extensive use, especially the 16-50, 60-250, and 17-70 zooms
  • The reliability of SDM lenses hasn't increased over time
  • The "budget" DC technology seems to be quite reliable, with no recorded failures
the problem is the mix bag with the SDM, some lenses hardly have any failures while others... i'm very curious what the differnce is between them.
It's also hard to say anything about the DC like you do now, it's only one lens afterall and no body know how it will hold out in another lens.

02-19-2012, 05:55 AM   #9
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QuoteOriginally posted by Anvh Quote
the problem is the mix bag with the SDM, some lenses hardly have any failures while others... i'm very curious what the differnce is between them.
Is it related to the weight of the element group being moved during focusing?
02-19-2012, 11:35 AM   #10
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QuoteOriginally posted by lytrytyr Quote
Is it related to the weight of the element group being moved during focusing?
I really don't know, I don't even know if the SDM motors that used are the same so it's very hard to say anything about the posiable cause.
02-19-2012, 01:34 PM   #11
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QuoteOriginally posted by Anvh Quote
I don't even know if the SDM motors that used are the same so it's very hard to say anything about the posiable cause.
I would think that the economics of production would encourage a common design (except for the mass-market "DC" in the 18-135mm).
02-19-2012, 02:53 PM   #12
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QuoteOriginally posted by Adam Quote
The survey can't really determine a percentage of failures due to response bias, but it can say this much:
  • SDM fails on lenses that see extensive use, especially the 16-50, 60-250, and 17-70 zooms
  • The reliability of SDM lenses hasn't increased over time
  • The "budget" DC technology seems to be quite reliable, with no recorded failures
quibbling a little, I don't see a basis in the data for a conclusion that the reliablility hasn't increased over time........ The data
shows a drop of about half from 40% to 20%. this could be a result of several possiblities
a) the large number of reports in 2011 is more statistically valid
b) they are newer and will fail soon
c) they are improved in relaiblity

What gives me optimism is that from looking at the board traffic here over the last 5 years, I get the distinct impression that a large portion
of the falures are either DOA or "D real soon" after purchase. I've seen multiple reports of folks who report returned 2 or 3 lenses before they found one that worked. Considering the total numbers reported in the survey (very small), and suspecting that these very unhappy people
were sure to respond in the survey....I'm inclined to surmise that the data is tilted toward DOA's (or near DOA), and I resist your conclusion
that it shows that every SDM out there works only because it hasn't been used enough to fail.
02-19-2012, 04:31 PM   #13
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QuoteOriginally posted by rvannatta Quote
quibbling a little, I don't see a basis in the data for a conclusion that the reliablility hasn't increased over time........ The data shows a drop of about half from 40% to 20%. this could be a result of several possiblities a) the large number of reports in 2011 is more statistically valid b) they are newer and will fail soon c) they are improved in relaiblity
Older lenses, e.g. purchased in 2009, are more likely to have failed by now because they've been used more often. So this by itself does not indicate that more recently manufactured lenses are more reliable.

Dan
02-20-2012, 07:16 AM   #14
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QuoteOriginally posted by dosdan Quote
Statistically, no difference. But it would make a difference to the user experience. Say 20% of the 2009 date-of-purchase lenses failed quickly (within 0.5yrs) and then stopped failing, compared to a more spread-out failing (after an avg. of 2yrs). At least the spread-out failure rate would allow the avg. affected user more use of his/her lens before failure. (And less chance of getting it fixed under warranty!)

Dan.
Dan,

I see what you're saying - not much different than what I said about knowing the amount of usage before failure. I just don't know how you get accurate info though.
02-20-2012, 08:28 AM   #15
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DC is WR, faster, cheaper, better and zero failure compare to SDM. What is the problem really to switch all motor to DC?
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