The standard deviation of this sample was 434 lbf.

You get a 95% confidence interval of 4822-5734. That means that you would expect that 95% of all quicklinks of this manufacture tested would fall within this range, and more importantly that 97.5% of all quicklinks would fail above 4822 lbf.

Tested at 3-sigma, the already very conservative standard used by most climbing manufacturers, you find that these quicklinks would get rated at 3974 lbf, or about 17.6 kn. You would thus expect that 99.87% of all of these quicklinks would fail at above 17.6 kn.

It seems to me that the safe working load printed on the links is ridiculously conservative. In fact, it works out to well over 8 sigma.

Edited to rework numbers.

Your calculations are incorrect and you've drawn the wrong conclusion from them.

The mean of the sample is 5278.2 lb, and the sample standard deviation is 434.7 lb. This is not the population standard deviation, only an estimate of it. Since the population SD is unknown, if we assume that the population is normally distributed, then the sample mean has a t-distribution with 5 degrees of freedom. The 95% confidence interval of the mean is equal to the mean +/- 2.571 SD, or 4160-6395 lb.

This gives us an indication of how precise our estimate of the population mean is, and not much else, since we do not have sufficient evidence evidence to conclude that the population is normally distributed. In fact, we know essentially nothing about the distribution. For instance, who is to say that 1 link out of 100 isn't simply pure junk and breaks at say 500 lb.

No, you are incorrect. The 95% CI at 5df is the mean +/- 2.571 * Standard Error

SE = SD/sqrt(n)

Yes, I am assuming that the there is a normal distribution of failure points, mostly because it's the same assumption that major gear manufacturers make. With more data, I can verify this. It's also a pretty standard assumption to make when you're doing things like this. However, you are correct in that I should state my assumption of normal distribution, and I have edited my posts to do so.

I'm sure I'm showing my statistical ignorance with the following questions...

How would you go about determining distribution?

Don't you have a similar issue with any manufacturing process that doesn't test every item made? What's to say 1 in 100 Petzl links won't fail at 500 lbs?

Our problem is that we're basing a decision on just 6 units. Legitimate manufacturers test thousands of units, so they have a very good understanding of the distribution of failure loads. In a properly controlled manufacturing processes the SD is quite small compared with the mean. Published independent tests of climbing gear have demonstrated this. When a legit manufacturer quotes a 3-sigma rating you can be confident that even the < 1/1000 unit that doesn't meet the 3-sigma standard will still have a failure point that is quite close to the standard (since the SD is small).

Our problem is that we're basing a decision on just 6 units. Legitimate manufacturers test thousands of units, so they have a very good understanding of the distribution of failure loads. In a properly controlled manufacturing processes the SD is quite small compared with the mean. Published independent tests of climbing gear have demonstrated this. When a legit manufacturer quotes a 3-sigma rating you can be confident that even the < 1/1000 unit that doesn't meet the 3-sigma standard will still have a failure point that is quite close to the standard (since the SD is small).

And yet well respected climbing manufactures have recently implemented recalls of defective equipment that has made its way into the hands of consumers.

Another thing, the manufacturer only declares a SWL of 1550.

It could be that they have tested 1,000s and due to variability are unable to state (based on statistical standards) anything more.

That doesn't make it a bad product per say, and it doesn't necessarily make it unsuitable for all climbing applications.

JT:
There are many ways to interpret a CI, all following different paradigms of statistics.

In these case, I am using the convention consistent with what has been stated by Black Diamond on their testing website.
http://www.bdel.com/about/3_sigma.php

It is not the only interpretation, nor is it the most technically correct, but it is arguably the most useful interpretation in this case.

Alternatively, you can state that you are 97.5% sure that the biners will fail after the lower confidence level. I believe this is the interpretation to which you are alluding. It is also a correct, albeit vague, definition.

Strictly speaking, if the failure of point quicklinks is in fact distributed normally, my interpretation will hold true and only 2.5% of quicklinks of that manufacture would be expected to fail above the lower limit stated.

Yes, it's true that I don't know if failure is normally distributed, but it's really the best assumption in this case since similar objects tested in similar ways have normal distributions of their failures.

Jay,
Im not familiar with the ratings on most 5/16 quicklinks, but you seem to be opposed to their usage in climbing. I have seen MANY quicklinks in action on permadraws and/or anchors.

What is your take on the general usage of quicklinks over biners?

RJ

On another note, we don't know the true population mean or standard deviation.

It is my understanding that if the true population standard deviation is unknown, you use t at df instead of z, which should yield an estimate of the CI for the population. Please correct and/or PM me if this interpretation is wrong.

The large standard deviation indicates that the quality control process is inadequate for use in a process on which lives depend. I've already stated that.



Yes, actually it does, and it does.

Jay

However you cant tell if your rap anchor will be only used for rappeling. what happens if someone uses it as protection, when linking pitches together and takes a big fall on it?

What happens if the next batch is made of steel from another supplier?
What happens if the online reseller gets the next batch from another manufacturer?
How does the manufacturing company make shure that they produce stuff of the same quality?

These quicklinks all broke at forces 5 times higher than they would realistically ever experience as part of a rap anchor.

Summary of statistical argument for those who don't care:

My calculation of the confidence interval was correct, but the interpretation of it was not. I maintain that whether or not you can assume normality is largely at the discretion of the user.

My apologies for poor tone used earlier.

However, I do wonder about the statistical signifance of the sample size relative to the box of 50 that I pulled them out of. How does that compare to testing 1000's out of millions produced?

isn't it about our statistical confidence? How do you know you #2 cam won't break during a normal fall?

If, after 1000 tests no device failed under 5000 pounds you can be pretty confident none would break at 3000 pounds.

If our quick links are highly variable we can still establish an expected range of failure and work within that. Could we not?

But the bigger issue here, which apparently I am the only one who understands, is we have no valid statistical reason to believe that the very next link you pull out of the box won't be junk.

I simply want to know two things
1. what is the average strength of the device
2. is that strength likely going to diminish quickly over time.
3. What are the forces I can expect to generate on that device.

We have no basis to rule out, for example, that the distribution is normal, except for 1 unit in 500 that is junk.

Another pertinent question is whether or not that "junk" unit is easily identifiable. I would bet that one of these units that would fail below the rated SWL would have visible and/or functional defects that would be obvious. But statistically, we have no groundts to predict that either.

Does anyone know of or have a report of a single quicklink that has broken in climbing applications?

I have heard of plenty of biners but never a quicklink.

Most of the gyms I go to have quicklinks on the bolt of the draw. These probably take more falls more often than anything out there. I don't think they're ever replaced.

I've still not heard of one breaking.

Jay

I realize that the discussion is on the quicklinks in question, not quicklinks in general.

For a good many years (most of technical climbing's history) quicklinks manufactured by reputable climbing companies have not been available.

These are extremely recent. I don't know if I've ever rapped off a petzl or fixe brand quicklink. I bet I've rapped off over 1000 different quicklinks. I had no clue of the history of them, who manufactured them, or the QC process involved.

Almost all other climbers are in the same boat. All over the world, not just in America.

Then gyms also use QL's and I've never seen one of them branded at all.

With the enormous use of quicklinks with all manner of variability in their quality, I hardly believe that Tradklime's are the only ones of dubious origin. I have yet to ever hear of a QL specific accident or failure. Ever.

I don't follow this sort of thing though, this is a legit question. So maybe someone who does follow this can tell me. Has a quicklink ever been broken by a climber?

You know... since there's not that many in a box, it wouldn't be too hard to proof them individually by breaking a loop of 6mm cord. That'd be enough to proof the individual piece to ~12kN and could be done with a couple long pieces of pipe or 2x4's (think levers...). It would also be low enough to not prevent later use.

These quick links are garbage. Let me guess: the OP doesn't want to pay for quality hardware.

Jay

I assume you are referring to me since I sent the subject quicklinks to Aric, and it's certainly

Frankly, with your reaction to this, I don't understand how you clip bolts that you didn't install yourself. And how do people justify TRing off of 2 quick draws at the top of a climb, when they could use lockers. And how do people justify only placing 3 pieces of gear for an anchor when they could place 10.

You may feel that this limited testing is sufficient to determine these quicklinks are junk...

It's a good idea, 4-5 kN is probably enough.

It's funny to me that up in your ivory tower you feel the need to selectively quote to be misleading.

In 20 years of posting on internet forums, listserves, and the like, I am proud to say I have never, ever done that. Perhaps you just didn't understand something for a change.

Edit: What is especially odd about your accusation is that I quoted your post in its entirety!

Jay

I don't know what Tradklime's situation is, so maybe the cost difference between the quicklinks vs new webbing plus rap rings is a factor.

Speaking of which, please pardon the personal interjection... Hey Tradklime- you around the 11th or 12th of May (Sunday/Monday)? Looks like my wife will be bailing on me for climbing while we're out in Boulder visiting relatives, so if you'd be interested in showing an out-of-towner around the Flatirons (or wherever, preferably trad) drop me a PM. I'll have the prototypes with me and promise not to replace any of your raps stations (while you're looking....).

-aric.

The equipment in question has a SWL of 1550 lb, which is greater than any expected load when used in the manner tradklime has in mind.

If I found one (or two) of these quicklinks at a rap station would I rap off them? Hell no. But I'm also the kind of guy that tends to cut off all the tat and completely replace everything at a rap station when I get to it.

We can debate the appropriateness of using them until the cows come home, but long story short we're seriously lacking in information about the quicklinks (materials, safety factors, etc.) and no amount of online bickering is going to change that. If the results I got makes Tradklime trust them for the use he intends, more power to him. If I find them and replace them with climb-spec gear, more power to me.

Holy fuck...In a similar vein, how do we know the head of your penis won't pop off from all of the pulling? How big was the sample size? No penis heads have ever failed before, but that does not mean that the head couldn't just pop off from work hardening....

I agree, that would make for an interesting comparison but no one I know of keeps a wrench on their rack so I have doubts as to the relevance of the results (unless it convinces people to keep a wrench on their rack for this purpose...).

What's there to get? Yes, I believe they're probably safe to rap off. I also clearly stated that I'm the type of guy that generally replaces everything regardless of what's there due to concern about the soft goods. If I'm going to go to the trouble to replace the soft goods I might as well leave the station with unquestionably good, climb-spec gear. If there was climb-spec gear there to start with, I'll just replace the soft goods and reuse the gear. If the gear isn't climb-spec, is in poor condition or is of unknown origin, I'll replace it. While I feel confident in my ability to make the determination of what's good enough because of my background in engineering and manufacturing, I don't know if the next guy will be similarly qualified and would prefer doing what I can to make it so that its not an issue for him.

Exactly why I do it.

My guess would be that the market for these sorts of things has considerable overlap between recreational climbing and uses that demand far stronger and more rigorously tested gear such as emergency services, search and rescue, military, law enforcement, etc. Its far cheaper to just make a lot of one product to the higher standard than smaller quantities of two different products with different design criteria.

True, it was an assumption on my part lead to only hand tightening them. I don't know offhand whether Tradklime intended to use a wrench or not as it wasn't discussed. As to what "most climbers do", I think neither you nor I are qualified to say what that is with any degree of authority. It wouldn't surprise me if someone were to use them in the fashion I envisioned though, and believe I've read about that very thing here on rc.com. Please feel free to send along some quicklinks for testing with the nuts wrench tightened, but until you do I'm not terribly interested in quibbling over the validity of the assumptions I made for the testing in a nonconstructive manner when they are, in fact, quite valid assumptions to have made regarding the use of the samples being tested.

Good thing I don't climb in JTree then, I suppose.

Well, off the top of my head OSHA regulations come to mind pretty quick. Plus I would think that law enforcement probably follows the guidelines set by the search and rescue folks, so perhaps someone like Majid could provide some insight on the how and why they spec their gear the way they do?

Actually, I'd contend that only occurs because CE and / or UIAA certification is required on equipment sold in Europe for use in rock climbing.

Jay, I'm afraid I have yet to see much of anything from you in this thread that wasn't argumentative, so my defensive reaction to your nitpicking my methods shouldn't be surprising.

And without specific information regarding torque specifications, how valid would the test be with tightening the nut with a wrench an arbitrary amount?

Given the uncontrolled nature environment that rap stations are in, the best way to assess suitability of the gear would be worst case scenario, which in this case would be having the nut only finger tight. That way the test would account for cases where a climber gets to the rap station and finds the quicklink undone and somehow manages to close the nut by hand.

Did you miss the part where I talked about the community at large maintaining things, thereby reducing the need for any one person to shoulder the entire load?

Personal attacks are never part of productive discussions and frankly inappropriate here in The Lab. I think I'll spend the rest of the afternoon amusing myself with the irony of using your own killfile tool on you.

And I might as well quote the whole thing for context and posterity....

...Or to see about having you banned from The Lab. Frankly you've added little value to this discussion...

I've PM'd a Mod with my complaint about it and the quoting was to prevent you from hiding the evidence.

...Or to see about having you banned from The Lab. Frankly you've added little value to this discussion and your behavior has clearly violated the TOS for this forum. I've PM'd a Mod with my complaint about it and the quoting was to prevent you from hiding the evidence...

Do you think that the only reason that BD and Petzl maintain high quality standards is because the CE and the UIAA says they must? I rather doubt it, especially since UIAA is a climbing industry organization. That is, the climbing equipment industry develops its own equipment standards (to which conformance is voluntary).

It still might me interesting to see how wrench-tightened links perform compared with finger-tightened ones.

1) n=6 is hardly a big enough sample to say with any certainty that these are "safe". Ever hear of bimodal distribution? Maybe 1 in 15 will fail at 500+/- lbs for some reason.

If you are truely interested, I would send some more to Aric, assuming he is willing to continue this...

1500 lb no name harware quicklinks are way better than the 2" shrub for rappelling of a route.

Unless your belayer is fooling around with you, 1500 lb backed up by a second bolt/quicklink on a top rope or rappel is quite solid to me. I would use it anytime.

I would be more concerned with noninspectionable bolts, and then only the first 2 anyway.

Nice testing though, proof that even cheap chinese no names are perfectly fine to rap off.

You should rap off the ones that actually say 300 kg on them before complaining about the 1500 lb ones.

No matter what anybody claims after the fact, let's face it: the reason that these links were sent for testing in the first place is that the owner didn't trust the quality of a 75-cent quick link made by an unknown Chinese manufacturer.

In spite of the fact that I have clearly explained this repeatedly, there is evidence that only one other participant in the thread has understood; while, in contrast, the majority of participants, including the owner of the links and probably the tester himself, believe that the testing has proved that these 75-cent links are safe.

Inference to the population from which the sample was drawn requires additional assumptions, whose justification is dubious... If quality control were good then the failure rates would presumably have an approximately normal distribution... As another poster said, the distribution could well be "bimodal," with a significant percentage of units being dangerously weak outliers.

Hey Scott,

You have a very good point here that unfortunately got lost in all the unpleasantness.

You won't see a reputable climbing gear manufacturer selling gear with an 8% coefficient of variation.

Could you post up a half dozen (or more) products with their coefficients of variation? It would be quite helpful.

"We pull-tested four of them [carabiners], seeing fractures of the gate notch at an average of 6100 pounds. Far more interesting than the strength value was the fact that the observed standard deviation was 70 pounds. The tightly grouped failure values indicate close manufacturing controls and dimensional tolerances."

This has to be the most pathetic thread of all time.

What happened to common sense. If you went to ace hardware and bought the cheapest quicklink on the rack and then used it to suspend the fattest guy you can find....

the quicklink wont break.

Does only testing four of them give us an accurate representation?

You know, I would have liked to see the information you so blithely maintain exists. Your one data point for a "reputable climbing gear manufacturer" is

Alexander Klimchouk, from Ukraine, has provided us with some incredibly well made titanium biners

Dated ~'91, whoever they were might be very good, but clearly don't meet your criterion.

If you are truely interested, I would send some more to Aric, assuming he is willing to continue this...

And neither will an Alien, I suppose.

Jay

I wouldnt suggest using a quicklink like an alien to protect a lead, but the original stated intent was for rap stations.

Can anyone come up with a remotely possible scenario where a quicklink gets loaded to 3000 or even 2000 lbf when used in a rap station.

Maybe if you use it as a belay anchor with the quicklink as the one and only tie-in point and then the leader does a 50 foot runout before falling directly onto the quicklink....

But that is just a herd thinning scenario.

I wouldnt suggest using a quicklink like an alien to protect a lead, but the original stated intent was for rap stations.

Can anyone come up with a remotely possible scenario where a quicklink gets loaded to 3000 or even 2000 lbf when used in a rap station.

Maybe if you use it as a belay anchor with the quicklink as the one and only tie-in point and then the leader does a 50 foot runout before falling directly onto the quicklink....

But that is just a herd thinning scenario.

So what you're saying then is that lacking any manufacturer information on torque specs from the manufacturer, relying on what you learned in parachute rigging school using somewhat similar gear in a completely different application from a completely different manufacturer is adequate for conducting a statistically valid analysis of these particular quicklinks in this particular application?

Please pardon the tone as I'm still rather displeased with you, but you seem to be behaving at the moment so my apologies. Its just that there's a large inconsistency in what you've been saying is and is not acceptable for valid testing and analysis.

Its pretty clear that getting manufacturer data would be rather unlikely in this scenario...

[snip stuff about torques]

I'm thinking it would be hard to properly torque a quicklink in the field anyway, since you cannot get a socket on them even if you did have a torque wrench.

Maybe someone makes some sort of open-end or box-end torque wrench.

I don't think the exact torque value is very critical to the strength of the link, as long as the threads are not damaged from overtightening. I think a 1/4 turn past snug would be just fine.

"Why might rappel anchor gear be judged according to higher standards than gear used for intermediate lead protection? Because the latter is a backup to the skill of the leader, and can itself be backed up. With a rappel anchor, it's all you've got; you're 100% reliant on the gear. This really should not have to be explained.

What is going on in this thread, thowever, is exactly the opposite of what is appropriate. The links are being judged by more lenient standards. You won't see a reputable climbing gear manufacturer selling gear with an 8% coefficient of variation."

So Jt, based on your statement above you are suggesting it would make more sense to use a quick link for lead than for rappelling? Tell me, how strong is the "skill of the leader"? Is it strong enough to resist the forces of a lead fall? Is it some sort of apoxy that can be smeared around the crux area to prevent device failure?

Has a quick link ever failed anyone on rappel?
S

Its 4,736.27 in California alone. More if you include Guam. Go look it up..... I suggest using Google rather than going and counting personally, although if you go that route we might actually get to discuss things without needing to put up with your attitude.

On a serious note, it absolutely is relevant to this discussion and you didn't answer the question.