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## Tests about a population mean

# Writing hypotheses for a significance test about a mean

AP.STATS:

VAR‑7 (EU)

, VAR‑7.C (LO)

, VAR‑7.C.1 (EK)

## Video transcript

- [Instructor] A quality control expert at a drink bottling factory
took a random sample of bottles from a batch and
measured the amount of liquid in each bottle in the sample. The amounts in the sample
had a mean of 503 milliliters and a standard deviation
of five milliliters. They want to test if this
is convincing evidence that the mean amount for
bottles in this batch is different than the target
value of 500 milliliters. Let mu be the mean amount of liquid in each bottle in the batch. Write an appropriate set of hypotheses for their significance test, for the significance test that the quality control expert is running. So pause this video and
see if you can do that. Now, let's do this together. So first, you're going
to have two hypotheses. You're gonna have your null hypothesis and your alternative hypothesis. Your null hypothesis is
going to be a hypothesis about the population
parameter that you care about and it's going to assume
kind of the status quo. No news here. And so the parameter that we care about is the mean amount of liquid
in the bottles in the batch. So that's mu right over there. And what would be the
assumption that that would be, the no news here? Well, it would be 500 milliliters. That's the target value. So, it's reasonable to say, well, the null is doing
what it's supposed to, that where the actual mean for the batch is actually what the target needs to be, it's actually 500 milliliters. Some of you might have said, hey wait, didn't they say the amounts in the sample had a
mean of 503 milliliters? Why isn't this 503? Remember, your hypothesis is going to be about the population parameter. Your assumption about
the population parameter. This 503 milliliters right over here, this is a sample statistic. This is a sample mean
that's trying to estimate this thing right over here. When we do our significance test, we're going to incorporate
this 503 milliliters. We're going to think about, well, what's the probability
of getting a sample statistic, a sample mean this far or further away from the assumed mean if we assume that the null hypothesis is true, and if that probability
is below a threshold, our significance level, then
we reject the null hypothesis and it would suggest the alternative. But if we're just trying to generate or write a set of
hypotheses, this would be our null hypothesis, and then
our alternative hypothesis is that the true mean for the batch is something different
than 500 milliliters.