What do you consider a sample to mean during an EICR

For sampling to be valid, it requires an adequate "population" to sample.

The minimum population size for a sample is 100. That's "things in general" - so it depends on what aspects of "inspection" or "testing" you are talking about - and what the risks of missing something are.

So, in a caravan for example, there's little room for missing out any inspections or tests, as a sample will be meaningless.

Although that presumably applies to circumstances where each test is independent. In real-life domestic situations, the condition of the wiring and accessories are likely to be highly correlated - all old or all new, all done by someone (in)competent etc. So if a few all pass, its more likely that all are good than would be expected if every socket had been independently installed.

wallywombat said:

Although that presumably applies to circumstances where each test is independent.

Well, yes ... and no ... what it means, is you need to be careful what and how you sample.

So, in dwelling, 20 % sample inspection of accessories on each circuit with more than 10 accessories (100 % otherwise), plus 100 % test of insulation resistance and protective conductor continuity might be considered OK, but not "20 % inspection and test" or indeed "20 % test" even if a separate 20 % is tested than the 20 % inspection sample.

But it's still important to note that, doing this, the socket-outlet behind the washing machine might never get sample inspected ... at least if you don't record what has been sampled each time, and the next inspector has the records ...

gkenyon said:

wallywombat said:

Although that presumably applies to circumstances where each test is independent.

Well, yes ... and no ... what it means, is you need to be careful what and how you sample.

I have got by in my working life knowing just enough about statistics.

A minimum population size of 100 for sampling seems very arbitrary.

I fully agree with ww - I&T of single items will only be reasonably independent if samples are taken at random and I think that this just isn't going to happen. The important thing is to avoid bias, which may be more or less obvious.

So perhaps you just "sample" sockets below 6 ft. It might be that a shorter electrician did them and somebody else did the higher ones, and if the shorter electrician's workmanship was the best, now we have bias.

Only a 100% "sample" can be 100% confident, but if say 9 out of 10 sockets pass, the maths will tell you how confident you can be that the whole of (the rest of) the installation is sound.

In the real world, I suspect that it is reasonable to conclude that if the first dozen accessories show excellent workmanship, the whole installation will be sound. By contrast, if the first half dozen are carp, you may as well conclude the EICR and recommend a total rewire.

For a ring, IMHO you should test all the sockets - it is quick and easy; but for a radial, if the end of the line is sound, you have probably demonstrated that the Zs and polarity are sound throughout.

Chris Pearson said:

Only a 100% "sample" can be 100% confident, but if say 9 out of 10 sockets pass, the maths will tell you how confident you can be that the whole of (the rest of) the installation is sound.

That's the point, with such a small sample size, and 9 out of 10 "passing", the maths doesn't provide confidence that 90 % of the installation is OK (or conversely, 10 % of the installation is potentially dangerous). But by the time the sample size is 100 or more, and you sample 90 % of those, then the confidence that the installation is sound increases.

However, inspecting 5 out of 10 sockets internally, checking terminals are tight etc, and testing them all, or testing a larger sample, say 7, that includes the ones you didn't inspect, helps improve the confidence. So, effectively, your inspection and test regime has the 100 % coverage based on the samples you inspect and test, if the "population size" is less than 100.

I think Lyle's earlier post sums up how he achieves this sort of approach (with an emphasis on inspection, which I agree is key).

Chris Pearson said:

Only a 100% "sample" can be 100% confident, but if say 9 out of 10 sockets pass, the maths will tell you how confident you can be that the whole of (the rest of) the installation is sound.

That's the point, with such a small sample size, and 9 out of 10 "passing", the maths doesn't provide confidence that 90 % of the installation is OK (or conversely, 10 % of the installation is potentially dangerous). But by the time the sample size is 100 or more, and you sample 90 % of those, then the confidence that the installation is sound increases.

However, inspecting 5 out of 10 sockets internally, checking terminals are tight etc, and testing them all, or testing a larger sample, say 7, that includes the ones you didn't inspect, helps improve the confidence. So, effectively, your inspection and test regime has the 100 % coverage based on the samples you inspect and test, if the "population size" is less than 100.

I think Lyle's earlier post sums up how he achieves this sort of approach (with an emphasis on inspection, which I agree is key).