STAT 6: Experimental Design

Question 1

What is the independent variable?

Answer 1

• also known as the explanatory variable
• the variable that causes an effect

Question 2

What is the dependent variable?

Answer 2

• also known as the response variable
• the variable that is affected by the independent variable

Question 3

What is error?

Answer 3

• the difference between the result for a whole population and the result from our sample or experiment

Question 4

What are the two main types of error in experimental design?

Answer 4

• sampling error
• bias

Question 5

What is sampling error?

Answer 5

• sampling error is the error caused by the possibility that the sample of neurons measured is not a perfect representation of the whole population
• we expect this error to be normally distributed
• we can use statistics to estimate it
• we can also take care in the design of the experiment to minimise the impact of sampling error

Question 6

What are the main techniques used to control for sampling error?

Answer 6

Replication:

• the more times you repeat an experiment, the more data you collect, so the more accurate the measurement become
• repeat experiments under the same conditions, but across multiple, independent experimental units
• they can be biological (testing in multiple different animals/patients) and technical (same conditions)

Balance:

• this means that it is best to compare groups of similar size
• comparing groups of different sizes may mean that their variance is more likely to be different
• t-test and ANOVA both assume equal variance
• unequal sample sizes and variances can also affect statistical power
• a reduction in power will increase false negatives

Blocking:

• grouping experimental units that have similar properties (e.g. age, gender, height etc)
• within each block, treatments are randomly assigned to experimental units

Question 7

What is bias?

Answer 7

• bias is the error caused by a systematic difference in our estimation of our sample and the whole population

Question 8

What are the main techniques for controlling bias?

Answer 8

• simultaneous control groups
• blinding
• randomisation

Question 9

What is a simultaneous control group

Answer 9

• a method to control for bias
• a group of subjects that are not exposed to the experimental treatment but are treated the same in all other ways
• the control group must undergoes the process at the same time as the experimental group

Question 10

What is blinding?

Answer 10

• a method of controlling bias
• neither the experimenters nor the patients know who got the placebo, or who got the treatment
• this helps to eliminate subconscious or conscious bias

Question 11

Describe randomisation

Answer 11

• subjects are randomly assigned the placebo or treatment
• this is so both conscious and subconscious bias can be avoided