The Statistics Involved in Experiments

Carl Hamblyn

SO_P

S_AR

Designing an Experiment

• An experiment is a set of observations made under conditioned controlled by the observer

• Experiments allow us to claim cause and effect (if correctly designed)

Components of Experiment

• Question• Variables• Design

• Replication• Blocking• Randomisation

Question

• Question• Variables• Design

• Replication• Blocking• Randomisation

• “The purpose of this experiment is to see if…

• … has an effect on…”

• E.g. “The purpose of this experiment is to see if adding nitrate solution has an effect on the yield of tomatoes at harvest time.

Question (try to be precise)

Variables

• Question• Variables• Design

• Replication• Blocking• Randomisation

Independent and Dependent

• Independent – the treatment• Dependent – the one you measure

Replication

• Question• Variables• Design

• Replication• Blocking• Randomisation

Not just once…

Components of Experiment

• Question• Variables• Design

• Replication• Blocking• Randomisation

Blocking (homogeneous groups)

Randomisation

• Question• Variables• Design

• Replication• Blocking• Randomisation

Experiment Group and

Control Group

Random allocation is important

Year 12 vs Year 13

• Comparison between two groups– need a control group. – need an experimental group

• Comparison within the same group– before and after. – with the treatment, without the treatment

Ideas and help

• BBC website (google BBC psychology test)• Census at School website