## 15.2 The need for making decisions

In research,
decisions need to be made about
*population parameters* based on
*sample statistics*.
The difficulty is that every sample is likely to be different
(comprise different individuals from the population),
and each sample will produce different summary *statistics*.
This is called *sampling variation*.

*statistic*) is likely to vary from sample to sample, because each sample is different.

However,
sensible decisions *can* be (and *are*) made
about population parameters
based on sample statistics.
For example,
to determine if a pot soup is ready to serve,
we don’t have to consume the whole pot of soup (the ‘population’);
a sensible decision can be made from a small taste (the ‘sample’).
Likewise,
in research
sensible decisions about the population parameter
can be made from the sample statistic.

To do this though,
the process of *how* decisions are made
needs to be articulated.
In this chapter,
the logic of making decisions is discussed.

To begin,
consider the following scenario.
Suppose I produce a standard pack of cards,
and shuffle them well.
The pack of cards can be considered a *population*.

*suits*: spaces and clubs (which are both black), and heart and diamonds (which are both red). Each

*suit*has 13

*denominations*: 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack (J), Queen (Q), King (K), Ace (A). Most packs also contain two jokers, but these special cards are not usually considered part of a

*standard*pack.

Suppose I draw a *sample* of 15 cards from the pack,
and notice that *all* are red cards.
How likely is it that this would happen simply by chance?
See the animation below.
Is that evidence that the pack of cards is somehow unfair, or rigged?

Getting 15 reds cards out of 15 seems very unlikely,
so perhaps you may conclude that the pack is unfair in some way.
But importantly, *how* did you reach that decision?
Your unconscious decision-making process may have worked like this:

- You
**assumed**, quite reasonably, that this is a standard, well-shuffled pack of cards, so that half the cards are red and half the cards are black. - Based on that assumption then,
you, quite reasonably,
**expected**about half the cards in the sample of 15 to be red, and about half to be black. You wouldn’t necessarily expect to see*exactly*half red and half black, but you’d probably expect something close to that. - But what you
**observed**was nothing like that:*All*15 cards were red. Since what you observed (‘all red cards’) was not like what you were expecting (‘about half red cards’), the 15 cards in my hand*contradict*what you were expecting, based on your assumption of a fair pack… so your assumption of a fair pack is probably wrong.

Of course,
getting 15 red cards in a row is *possible*…
but very *unlikely*^{5}.
For this reason,
we would probably conclude that the most likely explanation
is that the pack is not a fair pack.

You probably didn’t *consciously* go through this process,
but it does seem reasonable.
This process of decision making is similar to the process used in
research.

In fact, the probability of getting 15 cards

*of the same colour*(either red*or*black) is about 0.0001025%.↩︎