Good data collection is built on good samples. But the samples can be chosen in many ways. Samples can be haphazard or convenient selections of persons, or records, or networks, or other units, but one questions the quality of such samples, especially what these selection methods mean for drawing good conclusions about a population after data collection and analysis is done. Samples can be more carefully selected based on a researcher’s judgment, but one then questions whether that judgment can be biased by personal factors. Samples can also be draw in statistically rigorous and careful ways, using random selection and control methods to provide sound representation and cost control. It is these last kinds of samples that will be discussed in this course. We will examine simple random sampling that can be used for sampling persons or records, cluster sampling that can be used to sample groups of persons or records or networks, stratification which can be applied to simple random and cluster samples, systematic selection, and stratified multistage samples. The course concludes with a brief overview of how to estimate and summarize the uncertainty of randomized sampling.
1.5 - What Happens When We Randomize?
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From the course by University of Michigan
Sampling People, Networks and Records
34 ratings
University of Michigan
34 ratings
Course 4 of 7 in the Specialization Survey Data Collection and Analytics
Meet the Instructors
James M LepkowskiResearch Professor
Survey Research Center, Institute for Social Research
Hello, welcome back to our lectures on sampling people, records, and networks.
My name is Jim Lepkowski.
And in this lecture,
what we're going to do is continue what we've been doing before,
where we saw what it was like to sample at all, and do so randomly.
And in this lecture what we're going to be doing is looking at the issues of what
happens when we sample in terms of the consequence.
What do we get?
Especially, what do we get when we sample randomly?
And so, we're going to focus on these consequences here,
in our probability sampling realm.
We're going to need to use our imaginations as we do this.
And I'll where we need to use our imagination,
but it's not that hard for us to do.
We're going to have to imagine certain kinds of things
that we're never going to see.
Sort of the the counterfactual, the thing that we wonder about.
We made that decision and it went in one direction,
what would have happen if it'd gone the other?
Well, we'll going to imagine that here, except that here when we imagine it,
we can make a quite concrete with those other kinds of outcomes are.
And even come up with ways of understanding what those outcomes
would have been like.
But in any case, what we're going to do is talk about our selection process by
talking about a seven step process.
Where we're going to start out talking about a population, to frame,
to sample kind of an operation, when we're interested in the elements.
And then go on and talk about what happens when we have one possible sample,
as opposed to many possible samples.
So, our material is going to deal with a process of sampling, but
in more conceptual terms for the time being.
[COUGH] So, let's start with the following.
Step 1 from our process, our survey sampling process, that we looked at before
involves a population definition, a population specification.
Let's suppose this is the population.
This is the collection of all the faculty members in our list,
all the transactions that we're going to be dealing with.
And it's just a diagram to represent those, and
that's really the first step in defining that population and
there's even a tiny little blue e up there and that hand with one finger pointed up.
We're dealing with the elements in the population.
Well, we know right away that we cannot get all of those elements in a list.
We can get a frame, a possibilist, and that frame quite
possibly could be very close to being identical to the population,
but in a lot of practice it's not.
And so, this frame that's shown here now is step 2, getting the list that we're
going to use, the materials we're going to use for the sample.
This frame is not offset temporally, or physically that one is behind the other.
This is the overlap.
And the frame doesn't contain all the population elements.
That's the little bit of light blue along the upper edge there for the population.
Whereas the frame also contains some things that are not in the population,
that you can see the shadow there to the right and the bottom.
So, the frame is not a perfect representation, but
that's what we work with.
So, population to frame.
And then [COUGH] we have our list.
And from our list, this is a representation,
we've looked at this before, this is a more complete list of the faculty,
this is 150 of the 370 faculty from our example from the last lecture.
And in this particular case, that's the list that we're using.
And we don't know for sure whether we've missed some faculty who were newly
enrolled in the university, some others who have been removed from the list,
they've moved to other universities, they've passed on, whatever it might be.
There might be a mismatch between this list and
the actual set of faculty at the moment we're doing the sampling.
From that list, we draw a sample, that's the third step.
Okay, population, frame, sample.
That sample is a microcosm of the frame, and
we're going to do chance selections, right?
So, we're going to use something like a table of random numbers to draw that
sample from the frame.
And there's our random numbers, this is the full representation of a list that I
gave you just a little corner of when we were doing our example sample.
And it lead to a possible sample that had 20 cases and
there's the sequence numbers of those that were chosen that we did last time.
And I've added the incomes because now we've gone out and we've interviewed
these faculty and we've obtained their incomes and we've calculated the mean.
Way down at the bottom we can see the mean income.
Okay, that's the process we went through before.
Our process would involve population specification, a frame identification,
a random selection of a sample, and then computation of an estimate.
Those four steps.
And here we're on solid ground.
These are all things that we can physically represent.
But we also recognize that this sample [COUGH] and
the one mean that we've got probably isn't the true population value.
As a matter of fact, there's almost zero chance that that's the actual mean of
the population, because we got a sample of only 20 from 370.
There's some error,
there's some discrepancy between what we have in our sample and the population.
If we had done a census,
we'd done every one, then we would have the exact mean under this scheme.
So, we can imagine then that maybe we had gone back to our list, and
started in a different place, and tried a different sample.
We, instead of going down columns 1, 2, and 3, we started over in columns 10, 11,
and 12.
We went row-wise.
We used the random number table in a different way, and
we got a different sample and computed the mean.
Drew a different sample from the 370 now.
We'll put them back, there's another possible sample.
There's at least three possible samples.
There's, a matter of fact, given the scheme that we used for sample selection
in our illustration, there are billions of possible samples of size 20 from the 370.
And for every one of them we can compute the mean.
And now, we're getting to the fifth step in the process.
Because, as we think about this, there's billions and billions of them,
possible different samples drawn and for every one, there's a mean.
And for every one of those means we're using the same sample size,
the 20 different elements, there's just a whole lot of them there.
And we can actually count how many there are.
We will do this for simple random samples, we're going to count how many
possible simple random samples of size 20 we could get from 370.
And so, we've got the first sample, the second sample, the third sample,
going across the bottom.
And then, dot, dot, dot, going on up to the last sample.
And you see, I've actually got a little index there with a funny notation that
I'll explain later.
But that's the last sample.
There's a countable number, there's a finite number of these possible samples.
But what's interesting, is now we have a set of means.
Billions of them.
And there's a distribution to them.
There's a range of values.
There's a smallest mean and a largest mean.
There's a group of means that's in the middle 95%.
There's a group of means that's in the middle two thirds.
There's a group of means up in the upper 5%, and so on.
There's a distributional property, interestingly,
and we'll come back to this later.
Under probability sampling,
these means will give us a distribution across all possible means that is normal.
That normal distribution, I didn't mention this before, but it is bell shaped.
It's quite remarkable really, if you think about it,
that we would get that kind of distribution.
That distribution has variance.
It has spread.
So now, we're up to a fifth step.
We're thinking about this consequence of doing this.
This is the consequence of doing random sampling.
We got lots and lots of possible samples.
We're never going to look at all of them.
So, in concept, this is kind of a waste of time.
Unless we could do something where we would say, can I estimate the spread of
all those possible samples by not doing more than one sample but taking one?
And it turns out that there's a piece of algebra, that we won't do, that would be
more in our sampling theory class, a piece of algebra that gives us our sixth step.
We start out with a population, we specified a frame, we drew
a sample from it, then we realized, and we computed an estimate from it,
and then we realized that we could repeat that sampling estimation process again and
again a large number of times.
And we had then means for
every one of those samples and the spread of those means.
We can define it.
We could calculate it if we were to do all those possible samples.
But instead, it turns out that that variance across all possible samples
has a simplified expression, algebraic simplification,
that is such that we can calculate that from just one sample.
One sample and we get something called a standard error.
And that standard error is a measure of the spread
of those means across all possible samples.
Now, this is quite remarkable.
We can imagine that, but we don't have to go through it.
Somebody's all ready done the algebra for us and given us that formula.
We can calculate it, as I said.
We can get these standard errors and
calculate them in this sixth step that give us a number.
In this case, the number from our sample size 20 is 6.02, 6,
that's our standard error.
Now, there's a meaning to it that will help us understand the uncertainty of what
we've just done.
Because we've got one sample from billions, we know it's not the right one,
we know we're uncertain, but the standard error's going to allow us to specify,
not only, it's not just a number, it allows us to specify the uncertainty.
And that uncertainty can be reflected in also how we design the sample.
It turns out that as we change the sample size, now, this isn't for our own
illustration, but as we change sample size, those standard errors go down.
And that helps us understand that I can get better and better results,
this is what we'll talk about in the next lecture, by changing the sample size.
Increasing it.
I can get worse results by decreasing it.
Okay, but back to our seven step process.
We've got six I've outlined through standard error.
Here's the last one.
That standard error can be used to give us an uncertainty statement.
That uncertainty statement is typically called a confidence interval.
There we're getting a statement that involves an upper and
a lower range on possible values for the means in the population.
If that interval is wide, we have a lot of uncertainty.
If that interval is narrow, we don't have as much uncertainty.
It's a very valuable way of capturing, in one statement,
not only what it is we've estimated, the middle of the that interval, but
also how wide, how far apart it is, our uncertainty about the result.
And so, there's a calculation of a confidence interval.
A calculation of a confidence interval that is one that is something we'll
look at.
And in this particular case, for that sample size 20, way down at the bottom,
we won't go through the calculations, we'll explain some of the technical detail
as we go along, there's an interval there from 66 to 98.
So, our mean is in the middle, 78, 78.6 is in the middle of that range.
And this is saying that our uncertainty is
such that it could be from 66 to 98 95% of the time.
That interval's going to cover what the true population value is.
If that were narrower, we'd feel less uncertainty about this.
This is a pretty wide range.
Now, it may be sufficient for our purpose.
Not saying that it's not useful if it's still uncertain.
But narrower would be better.
So, there we have it.
Six steps to get to where we need to be.
1 is a population, 2 is a frame, 3 is a sample,
4 is an estimate, 5 is to imagine all possible estimates, 6 is to calculate
the variability of those and 7 is to write it in an uncertainty statement.
That's the statistical process that we're going to be doing.
Okay, so just a bit of a summary.
We're very close to talking about two important measures of results here.
On average is this process giving us the right thing?
We're going to talk about that in terms of bias.
And then secondly, how variable is it?
That's the uncertainty statement, the variance.
Our random process, we're using random digits applied to a frame
to generate our sample, and in theory, a large possible set of samples.
And we can measure the variance across all of those, and
only random samples allow us to do this.
We can make assumptions to get to this conclusion,
I'm not saying that we shouldn't make assumptions to get to it.
But this is the kind of the frequency, the counting way of thinking about the world.
It's not necessarily the best, it's not the most coherent, but
it is one that you can get a clearer idea about than some of the others.
And it involves a population and a sampling mechanism.
It does not involve a population distribution.
We've made no assumptions about the population itself.
We've only got the sampling mechanism and our outcome that is then manipulated
because of the sampling mechanism to give us an uncertainty statement.
All right, we need to explore this just a little bit more.
We can use these ideas now to make statements about
how good the sample actually is.
We can evaluate the quality of our samples.
That is very important.
Everybody wants to know.
We started out talking about this.
How did you get that number?
How did you ask the questions, and so on?
We want to know about the quality, how good the result is, and we're going to
deal with the quality of the representation side of our survey sample
by looking at this kind of framework in a little more detail in our next lecture.
Look forward to seeing you at that lecture.
Thank you.
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