# Not able to understand the intuition behind $z$-test, $t$-test

I am not able to understand and appreciate the basic intuition behind need of this whole hypothesis testing framework, different kinds of tests, looking up tables, significance level etc etc. Why can't below simple thing work?. Please let me know what is missing from my understanding:

Say we want to test if a particular thing roughly (within acceptable limit) holds true on a population or not, say content weight in packets of some product. Let's say expected weight is $w_0$. We can take as large a sample n as possible and find weight in each packet. If n1 packets weigh close to $w_0$ and n-n1 does not weigh close to $w_0$ then if n1/n is large enough, then we can say that the properly holds(i.e. weights of packets are within acceptable limits). Now, where is the scope of this whole hypothesis testing framework, consulting tables, graph plot etc etc.

• (1) Who has the time and budget to take arbitrarily large samples? (2) Your logic is completely circular: although you are trying to estimate $w_0$, but you base your procedure on monitoring a quantity that depends on $w_0$ itself. Might I suggest you search our site for threads on hypothesis tests and p-values and study some of them?
– whuber
Commented Jul 21, 2016 at 14:02
• If you have the population and can process it, then there's not much in hypothesis testing. For instance, if your question is whether the average length of matches in this matchbox is larger than in that matchbox, then it's a matter of measuring the matches and comparing the average. No z- or t-tests involved. Now, if your question is whether the average length of matches from this vendor is longer than from that vendor, then you're in trouble. You obviously can only obtain samples from each vendor, then have to make inference about the entire population Commented Jul 22, 2016 at 14:27
• To amplify whuber's first comment: the $t$-test was invented by Gosset precisely because it is often impractical to come up with "as large a sample n as possible". (Imagine if they had to use the entire batch of Guinness produced in a run just to check if that batch was of good quality...) Commented Jul 23, 2016 at 3:47

## 2 Answers

Say we want to test if a particular thing holds true on a population or not,

This does sound like the sort of thing people tend to use hypothesis testing for.

say mean of content weights in packets of some products

Well, in that particular kind of situation, you might be interested in a slightly different question than the usual hypothesis tests will tend to answer.

We can take as large a sample as possible and find mean of weight in that; Let's call it $w$.

Well let's be more careful to emphasize the distinction between random variable, observed values and hypothesized population parameters. Statisticians have a standard notation partly to make that clear.

Let the hypothesized mean weight in the population be $\mu_0$. Let the random variable representing the mean sample weight be $X$ and let the observed sample weight in our particular sample be $\bar{x}$.

Then error is $∣w−w_0∣/w$.

i.e. the statistic you're suggesting is $|\bar{X}-\mu_0|/\bar{X}$, and its observed value in our particular sample is $|\bar{x}-\mu_0|/\bar{x}$.

That's the absolute relative error, but for some reason you've computed the error relative to the sample mean (a quantity subject to sampling noise at the least) rather than the thing you're interested in $\mu_0$, which is not subject to noise, systematic error or anything else. Which is to say, if you're going to construct a statistic like that, $|\bar{x}-\mu_0|/\mu_0$ might often be a more obvious choice.

If this error is within acceptable limits,

Here's where things seem to be diverging.

If "acceptable" is based on some external standard (like "is my sample mean within 1% of $\mu_0$") where 1% has been judged by you to be close, you now appear to be answering a different question to "we want to test if a particular thing holds true on a population or not, ... mean of content weights in packets of some products"

That's not saying the question you're now addressing would be a bad one to answer, but if I understand you correctly, it's not quite the same question you started with.

then we can say that the property holds.

Well no, you can't. You have no basis on which to say the population mean is $\mu_0$. Only that the sample mean happened in this instance to be close (in a particular sense) to it. Let's say your criterion of acceptability is the absolute error is under 1% and you observe a sample mean that's 0.8% above that hypothesized mean. Further, let's say that the actual population mean is a little above the population mean (maybe it's 0.68% above, say, but it doesn't matter, because the sample mean tells us our best estimate of it). In a sufficiently large sample you will be able to tell that the population mean is different from $\mu_0$ (if the error of 0.8% is larger than you could reasonably expect to see, given random sampling from a population with mean $\mu_0$).

Now, where is the scope of this whole hypothesis testing framework

Answering instead the question you started with.

Now there is a particular kind of hypothesis test related to questions about whether a population mean is "within acceptable bounds" of a specified amount (which sounds like what you need for this situation). That's called equivalence testing. What your own framework leaves out (but equivalence testing does not) is that to be reasonably confident the population mean lies inside the acceptable range you will need to sample mean to be well inside it (since the sample mean can diverge from the population mean).

Statistical ideas can help you figure out how far inside you would need to be for that.

• Thanks many Glen_b. I loved your way of answering -- analysing each line. I have editted my question to include things which I had meant but was not careful enough to include. Could you please re-read my question and add if you have anything new to add. Thanks again! :) Commented Jul 22, 2016 at 13:53

[EDIT: After the original question was edited, this answer might not hold anymore.]

I believe by "expected mean weight is $w_0$" you mean to say that your hypothesis is: "The mean weight is $w_0$". Otherwise, as @whuber stated, this doesn't make any sense.

Firstly, I believe we should stick to the case in which our sample size $n$ is smaller than the total population size $N$. If $n=N$, we don’t need any statistical inference – we can simply compute everything that we are interested in, like the mean and we will be sure that this is the true mean of the population. Using your terminology: No matter how small the error is – if it’s greater than zero, we would say the property does not hold.

Now let’s focus on the case, in which our sample size $n$ is smaller than the population size $N$ and, as @whuber stated, $n$ will in practice not be arbitrarily large. Say, the weights of the packages from your example are normally distributed with mean 100 and standard deviation 25 and your sample size is 25. You want to test the hypothesis that the true mean equals 95. The first problem you might face is defining a proper cutoff point for your „error“ (I will not comment on your choice of "error" , but taking it as it is, for this cutoff point to be in any way meaningful, it would also have to depend on $w$). But let’s say you were able to define a cutoff point. Now: The probability, that your error will exceed your cutoff point even though $w = w_0$, will in general be $> 0$, as will the probability that the error will not exceed your cutoff point even though $w \neq w_0$. So, if you are unlucky with your sample, even if you define a meaningful cutoff point (which will be a big problem), your approach will lead you to reject/not reject hypothesis that are right/wrong AND you have absolutely no clue as to how often any of these events happen.

Therefore, the classic hypothesis testing framework uses different tests, significance levels, powers, etc to give us tools to determine ourselves: What is the nature of my hypothesis and the nature of my problem? (Different tests) How big of a chance to „make a mistake“ will I accept in my test? (Significance Level, Power) etc. In other words: the classical framework has figured out certain „errors“ and cutoff points with respect to the nature of a given hypothesis, the data and your tolerance for making a mistake.

So to sum it up: I believe what your approach is missing, is taking into account that your sample might not represent the overall population very well. Additionally, it will not be easy for you to define a "meaningful cutoff point". While both your approach and the classic framework will not always reject a false hypothesis or keep a true hypothesis, with the classical framework you have tools to control for these occurences.

• Thanks for taking the time to write such a lengthy reply to enable me to understand the concept, but it's still not very clear. Could you please read my question again. Commented Jul 21, 2016 at 15:03
• I read your question again ("Why can't below simple thing work?") but still feel like I answered it. Could you please tell me what is still unclear?
– elmo
Commented Jul 21, 2016 at 15:08