Be careful of how you set up your 2.2 table here.

Cross, the cross product formula only works if the table is set up correctly.

For this epidemiology MOOC, we will use the convention

of disease on the top, and exposure on the side.

However, outside this course, you may see these switched.

As I said before, in a case control study, the odds ratio is the exposure odds ratio,

which is the odds of being exposed in the cases equals a divided by c.

Divided by the odds of being exposed in the controls or b divided by d.

Mathematically, this is the same as the cross product, which

is equal to a times d divided by b times c.

The odds ratio is the ratio of the odds

of the health outcome or disease in the exposed

Relative to the odds of the disease or health

outcome in the none exposed or less exposed group.

Odds ratios can be calculated in cohorts studies and in case control studies.

Prevalence odds ratios can be calculated for cross sectional studies.

There are different ways that you can interpret

a measure of association and words as illustrated here.

You could say ,those in a traffic ac,

accident were 1.62 times as likely to have been

texting while driving compared with those who were

not in a traffic accident in the past year.

Or those in a traffic accident were 62%

more likely to have been texting while driving.

Then those who were not in a traffic accident in the past year.

But the most precise interpretation is as follows the odds of a traffic accident

among those who texted while driving was 1.62 times the odds of a traffic accident.

Among people who did not text while driving.

Be careful, you cannot calculate a risk or rate directly from case control data.

The denominators obtained in a case control study do not represent

the total number of exposed and

non-exposed person in the source population.

The investigators arbitrarily decide how many controls

will be selected to compare with the cases.

We cannot directly measure risks or rates because the population at risk in the

denominator is not ascertained. Under certain conditions, we can

obtain a valid estimate of the rate ratio risk ratio using the odds ratio.

But we won't go into those conditions for this MOOC.

So let's review.