1. Turnbull High School Advanced Higher Biology Unit 3 P96-97
Investigative Biology Control groups, Observational Studies and Experimental Design
Turnbull High School
Advanced Higher Biology
Unit 3
P96-97
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2. Control Groups
In any scientific experiment, a control group should always b used. This allows you to compare the “treatment” with control results so the experiment can be shown to be valid.
There are two types of controls.
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3. Negative Control
The negative control group provides data for what happens in the absence of treatment.
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4. Positive Control
A positive control provides data for what happens in the presence of treatment – but not the one you are currently testing.
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5. Dependent Variable
A dependent variable is a variable that is measured to determine if changing the independent variable has an effect.
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6. Observational Studies
Sometimes it can be hard for investigators to control the independent variable.
For example, a scientist studying the effect of smoking on heart disease would not ask a group of people to take up smoking, just so that he or she could watch what happens.
Sometimes, observational studies occur using groups which already exist, at the expense of controlling the independent variable.
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7. Observational Studies
In observational studies, scientists look at correlations (relationships) between variables.
Positive correlation – when one variable increases, the other also increases
Negative correlation – when one variable increases, the other decreases.
It can be hard to prove that correlation results in causation – i.e. does increasing the number of cigarettes smoked increase the heart disease caused?
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8. In Vitro and In Vivo
In biology, all studies fall into two categories – in vitro or in vivo. In vitro – in glass e.g. in a test tube In vivo – within the living e.g. inside a person
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9. Advantages and Disadvantages of In Vitro studies vs In Vivo studies
Type of study
Advantages
Disadvantages
In vivo
Provides data for effects in whole organisms
Allows study of complex interactions
Expensive and time consuming
Ethical concerns
Difficult to control confounding variables
Results may be difficult to interpret
Difficult to prove causation
In vitro
Simpler and less expensive
Easier to control confounding variables
Interpretation of results is simpler
Can demonstrate correlation and causation
Difficult to extend results to whole organism or different species
Difficult to model complex interactions
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10. Evaluating Experimental Design
Can often be complex
Questions to consider:
“Is the experimental approach appropriate for this aim?”
“Are the controls suitable?”
“Are confounding variables adequately controlled?”
“Is there selection bias?”
“Is the sample size suitable?”
Miss A. Aitken