1. n The Experimental procedure involves manipulating something called the Explanatory Variable and seeing the effect on something called the Outcome Variable or the Response Variable. n Many times it is hard to establish a clear causal connection between one variable and another, it may be that a third variable is causing both. n There is an established correlation between the number of fillings in children’s teeth and their vocabulary. Chapter 5 Experiments and Observational Studies

2. n Does this mean that eating Mars bars increases your vocabulary?? n There may be a third variable causing both of the variables we are studying. n Ideally we want to create changes in the explanatory variable and then examine the effects on the response variable. This we can only really do in an Experiment. n In an Observational Study we cannot create changes in the explanatory variable. Instead we observe differences in the explanatory variable and try to see if these are related to changes in the response variable.

3. n So in an Experiment we have an element of control that we do not have in an Observational Study. n Why don’t we just do Experiments then? n Well it may be unethical to perform certain experiments. u Eg To measure the effect of smoking during pregnancy on a child, it would be unethical to make a random selection of mothers smoke. We could however observe the effects on the children of mothers who already are smoking during pregnancy. n It may be that we cannot randomly assign some explanatory variables- like baldness, or handedness.

4. Some Definitions n A Treatment is one or more explanatory variables assigned by the experimenter. n A Confounding Variable is one whose effect on the response variable cannot be separated from the effect of the explanatory variable. n An Interaction occurs when the effect of one explanatory variable depends on what’s happening with another explanatory variable. n An Experimental Unit is the smallest object to which we can assign different treatments in an experiment.

5. Some More Definitions n An Observational Unit is the smallest object which we can observe in an observational study. n When the Units are people they are called Participants or Subjects. n Usually these participants are Volunteers.

6. How to design an Experiment n Randomisation is the most important element of any experiment. We will discuss different types of randomisation in a little while. n A Control Group which is treated identically in all respects to the group receiving the treatment except that the members of the control group do not receive the treatment. n Placebos: There is a proven phenomenon called the placebo effect. Patients receiving Placebo tablets which have no active drug ingredient (eg a sugar tablet) may experience a certain beneficial effect.

7. n The way to eliminate this placebo effect from the experiment is to give Placebo tablets to the control group when administering a tablet to the treatment group. n Blinding: It is not just in receiving tablets that the “power of suggestion” plays an important role.It is usually best therefore if the subject does not know whether they are receiving the treatment or not. This practice is called Blinding. n Sometimes it is also best if the experimenter does not know which subject is receiving the treatment and which is not. This will remove any potential bias in the way the experimenter reports his findings.

8. n Experiments in which both the subject and the experimenter do not know who receives the treatment are called Double Blind. n Experiments in which either the subject or the patient (but not both) do not know who receives the treatment are called Single Blind.

9. Experimental design n The design of an experiment is very important. Experiments are designed with the purpose of isolating the effect of the treatment on the response variable and removing any confounding effects. n One way that we have seen already of removing the effect of any confounding variables is to randomly assign subjects to the treatment or control group. This way any possible bias in the population should be evenly spread among the treatment and control groups.

10. n Sometimes instead of relying on randomisation to make the groups as even as possible we actually force the groups to be similar. n Matched Pair designs: These are experimental designs in which either the same individual or two matched individuals are assigned to receive the treatment and the control. In the case where an individual receives both the treatment and the control, the order in which this happens should be random. And the experiment should be conducted as a Double Blind experiment.

11. n Block Designs: This is an extension of the Matched Pair design to the case of three or more treatments (one may be the control). If there are 4 treatments and a control then there will be 5 blocks each one designed to be as similar as possible. 4 of the blocks will each receive one of the treatments and one block will be a control.

12. Problems with Experiments n Confounding variables - u These are variables connected to the explanatory variable which may be the actual cause of the effect on the response variable. u Cured by Randoomisation u Storks n Interacting variables u A second variable which interacts with the explanatory variable. u Smoking/Alcohol/Exercise n Placebo effect n Hawthorne effect u Participants in an experiment respond differently just because they are in an experiment.

13. Problems with Experiments n Experimenter effect u The experimenter can influence the results of the experiment. u They may record data incorrectly. u Or inadvertently let the subjects know the desired outcome. u RATS n Ecological validity and generalisability u Results obtained in a closed experimental setting may not be applicable in the real world

14. Observational Studies n Observational studies are not as good as Experiments at establishing causal connections. n However since no special setting is required they usually do not suffer from the problem of Ecological Validity. n In addition the Hawthorne and Experimenter effects are not a problem.

15. Observational Studies n Observational studies are classified as either Retrospective or Prospective Studies n In Retrospective studies the participants are asked to recall certain past events. n In Prospective studies the participants are followed by the researcher into the future and events are recorded. n One particular type of observational study has become very popular n - The Case Control Study

16. Case Control Studies n To try and examine the possibility of a relationship between an explanatory variable and a response variable the researcher selects a group of participants called CASES in which the response variable is present. n For example in a study to determine if there is a relationship between baldness and heart attacks, a group of patients in hospital for heart attacks are chosen as the Cases. n A group who have not had heart attacks are chosen as the Controls.

17. Case Control Studies n The Control Group should be chosen to be as similar in all respects as the case group except for the response variable. Why? n A Case Control study is much more efficient than some other forms of study. Consider first choosing two groups according to whether the explanatory variable was present or not then waiting until the response variable revealed itself. This may take a long time. n Good at removing confounding variables if controls are chosen appropriately.

18. Problems with Observational Studies n Confounding Variables n Cases and Controls not representative of the population n Recollections of the Past may not be accurate.