# How to calculate sampling error?

Given a reservoir of size $S$ with each element taking a value of error or not an error, we attempt to estimate the number of errors inside the reservoir through the following

We poll the reservoir with $P$ samples, and verify that each sample is not an error (note this is ad hoc observed), we do this polling process $X$ trials

since the probability of $n$ error (set to some percentage of $S$ ) appearing in a trial is

$$\approx {n \choose P} (\frac{n}{S})^n (1 - (\frac{n}{S})^{P- n})$$

If the probability that no error appear in $X$ trials is low assuming there are $n$ errors in the reservoir, than we can assume that our observation of no errors guarantee that $n$ is fairly few in the resevoir

$$P(no \:errors \: despite \: n \: errors \: exist) \approx (1 - \sum_{n= 1}^{n}{n \choose P} (\frac{n}{S})^n (1 - (\frac{n}{S})^{P- n}))^X$$

If $P(no \:errors \: despite \: n \: errors \: exist) << 1$ than the fact that we observe no errors means that $n$ is few

but what I found out is that

$$P(no \:errors \: despite \: n \: errors \: exist) = 1$$

wolfram calculation

This is counter intuitive, since if we set $n = 350$ , polling an error out of the reservoir of size $S = 70000$ with $P = 1$ have a probability of $0.005$ so polling at least 1 error out of 500 polls must be greater than $0.005$

Can someone point out where I made a mistake?

• Your equations are inconsistent. What is $n$ in each case? Do you consider sampling with replacement? – d.k.o. Jul 9 '15 at 1:55
• n is the number of errors we assume to exist (this is for all cases) and I was calculating using sampling with replacement – user411754 Jul 9 '15 at 2:25
• Then this formula does not make sence: $$\approx {n \choose P} (\frac{n}{S})^n (1 - (\frac{n}{S})^{P- n})$$ – d.k.o. Jul 9 '15 at 2:42
• In the second eq. you are summing up over $n$ from $1$ to $n$... – d.k.o. Jul 9 '15 at 2:43
• ${n \choose P}(\frac{n}{S})^n(1 - \frac{n}{S}^{P-n})$ account for all n errors occur in the sample, it doesn't account for $n - x$ errors where $x \leq n$ – user411754 Jul 9 '15 at 5:06

If I correctly understand the described procedure we have a population of size $S$ and the proportion of errors in the population is $p=n/S$. To estimate $p$ (or $n$) you repeat $M$ times (with replacement) a random sampling of $P$ items (without replacement) and count the number of errors in each sample.

Let $\{X_i,\dots,X_M\}$ denote the number of errors in each sample of size $P$. Then assuming $S$ large enough relative to $P$

$$P\{X_i=k\}\approx \binom{P}{k}p^k(1-p)^{P-k}$$

and

$$P\{X_1=0,\dots,X_M=0\}\approx (1-p)^{P\times M}$$

You may want to estimate $p$ from the data $\{X_1,\dots,X_M\}$, e.g. using maximum likelihood. In this case (using binomial approximation again)

$$\ln\mathcal{L}(p|X_1,\dots,X_M)=\sum_{i=1}^M\binom{P}{X_i}+\ln p\sum_{i=1}^MX_i+\ln(1-p)\sum_{i=1}^M(P-X_i)$$

Maximizing $\ln\mathcal{L}$ over $p$ yields the MLE

$$\hat p=\frac{1}{M}\sum_{i=1}^M\frac{X_i}{P}$$

Now you can test (statistically) whether $p$ (or $n$) is close to $0$ or not.

• Intuitively, the final equation doesn't quite make sense to me, since $$\hat p=\frac{1}{M}\sum_{i=1}^M\frac{X_i}{P}$$ seems to say that when all $X_i$ are zero the probability of getting an error is zero, despite $M$ might be large, should the probability be normalized by the reservoir size? For example if you sampled 2 sample out of 10,000 samples , the fact that 2 samples are not error does not connotes that the global $p = 0$ but rather the local $p = 0$ and that this local $p$ have $1 - 2/10,000$ chance of not being correct, maybe I am getting something wrong – user411754 Jul 9 '15 at 15:18
• No, it should not be normalized. This $\hat p$ maximizes the likelihood of observing particular sample. It may be zero as well despite the fact that the true $p>0$. This is called "estimation" in statistics. If you want to know the true $p$, you must examine the entire population. – d.k.o. Jul 9 '15 at 18:34
• ah, that makes sense, is there also a way to measure the confidence of the estimation? – user411754 Jul 9 '15 at 20:17
• @user411754: Actually, this is equivalent to drawing 1 sample of size $M\times P$ if $MP<< S$ (so that binomial approximation to hypergeom. works): onlinecourses.science.psu.edu/stat504/node/36 – d.k.o. Jul 9 '15 at 20:35