What Is an Experiment?

An experiment is simply the test of a hypothesis.
   Experiment Basics
The experiment is the foundation of the scientific method, which is a systematic means of exploring the world around you. Although some experiments take place in laboratories, you could perform an experiment anywhere, at any time.
Take a look at the steps of the scientific method:   
     ·        Make observations.
     ·        Formulate a hypothesis.
     ·        Design and conduct an experiment to test the hypothesis.
     ·        Evaluate the results of the experiment. 
     ·        Accept or reject the hypothesis.
     ·        If necessary, make and test a new hypothesis.
  Types of Experiments
      1.     Natural Experiments

A natural experiment also is called a quasi-experiment. A natural experiment involves making a prediction or forming a hypothesis and then gathering data by observing a system. The variables are not controlled in a natural experiment.

       2.     Controlled Experiments

Lab experiments are controlled experiments, although you can perform a controlled experiment outside of a lab setting! In a controlled experiment, you compare an experimental group with a control group. Ideally, these two groups are identical except for one variable, the independent variable.

      3.     Field Experiments

A field experiment may be either a natural experiment or a controlled experiment. It takes place in a real-world setting, rather than under lab conditions. For example, an experiment involving an animal in its natural habitat would be a field experiment.

Variables in an Experiment
Simply put, a variable is anything you can change or control in an experiment. Common examples of variables include temperature, duration of the experiment, composition of a material, amount of light, etc. There are three kinds of variables in an experiment: controlled variables, independent variables and dependent variables.

       1.     Controlled variables,
 sometimes called constant variables are variables that are kept constant or unchanging. For example, if you are doing an experiment measuring the fizz released from different types of soda, you might control the size of the container so that all brands of soda would be in 12-oz cans. If you are performing an experiment on the effect of spraying plants with different chemicals, you would try to maintain the same pressure and maybe the same volume when spraying your plants.
      2.     The independent variable
The independent variable is the one factor that you are changing. I say one factor because usually in an experiment you try to change one thing at a time. This makes measurements and interpretation of the data much easier. If you are trying to determine whether heating water allows you to dissolve more sugar in the water then your independent variable is the temperature of the water. This is the variable you are purposely controlling.
      3.     The dependent variable
The dependent variable is the variable you observe, to see whether it is affected by your independent variable. In the example where you are heating water to see if this affects the amount of sugar you can dissolve, the mass or volume of sugar (whichever you choose to measure) would be your dependent variable.
Any problems with
     ·        Experimental Designs?
     ·        WHAT? Problems? What Problems?
      ·        Threats!!!
      ·        You threatening me?
     ·        Er, no, threats to internal and external validity
    4.      Internal Validity
– History, maturation, testing, instrumentation, statistical regression, differential selection, mortality
        5.      External Validity
– Pretest-treatment interaction, multiple treatment interference, experimenter effects, reactive arrangements (hawthorn effect).

Quantitative and Qualitative Data
Some methods provide data which are quantitative and some methods data which are qualitative.  Quantitative methods are those which focus on numbers and frequencies rather than on meaning and experience. Quantitative methods (e.g. experiments, questionnaires and psychometric tests) provide information which is easy to analyze statistically and fairly reliable.  Quantitative methods are associated with the scientific and experimental approach and are criticized for not providing an in depth description.
Qualitative methods are ways of collecting data which are concerned with describing meaning, rather than with drawing statistical inferences.  What qualitative methods (e.g. case studies and interviews) lose on reliability they gain in terms of validity.  They provide a more in depth and rich description.
Quantitative methods have come under considerable criticism.  In modern research, most psychologists tend to adopt a combination of qualitative and quantitative approaches, which allow statistically reliable information obtained from numerical measurement to be backed up by and enriched by information about the research participants' explanations.
You will find that many of the core studies do collect both types of data.