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What is the difference between a quasi experiment and a natural experiment?

Neuralizer

Most of the A level Psychology text books don’t explain the difference between the terms quasi and natural experiments and similarly neither do the specifications (AQA, Edexcel, OCR and WJEC) as it is slightly complicated.

Occasionally I get asked as a teacher about the difference between quasi and natural experiments and below is my answer.  I do hope it makes sense and if it doesn’t just use your neuralyzer to erase any memory of reading this.  I do believe though that it helps to know the difference between a quasi and natural experiment.

A true experiment is one where participants can be randomly assigned to the conditions of an experiment.  True experiments include laboratory and field experiments.

A quasi (quasi means almost) experiment is not a true experiment as the researcher is not able to randomly allocate participants to different conditions of the experiment.  This is usually because the independent variable is a quality of the participant.  For example, an experiment with an independent variable of gender is described as a quasi experiment as the researcher cannot randomly allocate participants to either the male or female condition. 

Other examples of a quasi experiment include testing people with autism and Down’s syndrome for their ability to understand other people’s emotions, or comparing older and younger participants’ ability to solve word search puzzles.  In both of these examples the researcher cannot randomly allocate participants to one of the two conditions as the independent variable is a quality of the participant.

Quasi experiments are often carried out under controlled conditions and therefore share almost the same strengths and weaknesses of laboratory experiments.  For example they may be highly controlled studies with low ecological validity.

However a weakness of quasi experiments is that because the allocation to the independent variable is not completely random we cannot be certain that the independent variable has caused any change to the dependent variable.   

Therefore quasi experiments are said to be lower in internal validity than a well controlled laboratory experiment.   However it is possible to improve the internal validity of a quasi experiment by increasing the size of the sample therefore reducing participant differences or using a comparison group who are very similar. 

Quasi experiments are also often known as difference studies. 

Quasi experiments are different from natural experiments in that they do involve a planned independent variable. 

 A natural experiment is a study of the effect of an independent variable, which has not been planned or manipulated by the researchers, on a dependent variable.

The word ‘natural’ in the term natural experiment therefore refers to an event that is not planned by the researchers. 

In a natural experiment the independent variable occurs because of some unplanned event.  That is, the independent variable has occurred because of some naturally occurring event and the researcher is taking advantage of this.

For example, researchers might be interested in how parenting styles influence the mental health of children and they could take advantage of two different cultures that use different parenting styles and then measure the differences in the mental health of children.

Natural experiments are useful because ethically and practically they are often the only design that can be used.   Further strengths of most natural experiments are that participants are often unaware that they are taking part in an investigation and that natural experiments will not be as artificial as laboratory experiments.

However natural experiments are much lower in internal validity than a laboratory experiment.  With natural experiments it is harder to establish causal relationships because the independent variable is not being directly manipulated by the researcher and there are so many other variables that could be affecting the dependent variable. 

The word ‘natural’ in the term natural experiment therefore refers to an event that is not planned by the researchers. 

Both natural and quasi experiments are not classed as true experiments because they lack either the research control (natural experiments) or random assignment (quasi experiments) of true experiments.

The differences between quasi and natural experiments though are sometimes somewhat less clear.  For example, if researchers implemented two different teaching styles in two different primary schools to measure the impact on learning this would be classed as a quasi experiment as the independent variable is planned.  However if researchers measured the impact of learning in two different primary schools which was not planned or had been implemented by the researchers this would be a natural experiment.

Importantly both natural and quasi experiments have a weakness in terms of demonstrating causation compared with a true experiment.

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