1. Logistic Regression we will go through the pdf first to outline some terms refer to earlier ppt on ODDS RATIOS (Stats Club)

2. Cause and effect Relationship between a risk factor and an outcome
Risk Factors –increased/decreased liklihood/probability that the disease will develop later
Comparison of risk between the animals exposed and not exposed
Causal relationships are complex

3. Odds ratios vs relative risk

4. ODDS
The chances or likelihood of something happening or being the case.
Ratio between the odds of disease in exposed animals and the odds of disease in non-exposed animals
OR= Ratio between the odds of disease in exposed animals and the odds of disease in non-exposed animals
OR= odds of being a case in exposed group
odds of being a case in non-exposed groups

5. So......... Odds of disease/case in exposed =a/a+c = a/c c/a+c
Odds of disease/case in non-exposed = b/b+d =b/d
d/b+d
OR = a/c or OR = ad
b/d cb
Exposed
Not-exposed
Disease +ve a b
Disease -ve c d

6. Values Values always >0 1 – no difference
<1 – exposure is protective
>1 – exposure increases odds of disease
Confidence intervals

7. Relative risk
Increased or decreased risk of disease in animals with an exposure of interest
Risk is the potential that a chosen action or activity (including the choice of inaction) will lead to a loss (an undesirable outcome). The notion implies that a choice having an influence on the outcome exists (or existed). Potential losses themselves may also be called "risks". Almost any human endeavour carries some risk, but some are much more risky than others.

8. So......... Risk of disease in exposed=a/a+c
Risk of disease in non-exposed = b/b+d
Relative risk =a/(a+c)
b/(b+d)
Exposed
Not-exposed
Disease +ve a b
Disease -ve c d

9. Look for…
The point estimate, the P value and the confidence interval – you want the actual numbers not ‘95% confidence’
Cases were significantly more likely to have ever have received a vaccine of any type in their lifetime compared to controls cats (OR = 6.8, 95% CI = , p = 0.03).

10. 2. Interpretation
The upper and lower limits can be used to see whether the results are useful
A value can be significant with a low p value but the CI interval can help tell you whether you should get excited about it or not!

11. The null value?
In Odds Ratios and Risk Ratios where you compare two groups and a value of 1 means there is no difference then 1 is the null value
If 1 is included in the CI e.g , then there is no statistically significant effect

12. Practical
Open word file ‘practical 2 solutions’
We will work through the file (or you can do in your own time) using first SPSS and then Genstat.