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Experimental Design

Experimental Design. Procedure. The written step-by-step plan for an experiment. This plan states what you will do at each step and what materials you will use. Variable. A variable is any factor that can affect the results of an experiment. The variable is what changes. Example.

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Experimental Design

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  1. Experimental Design

  2. Procedure • The written step-by-step plan for an experiment. • This plan states what you will do at each step and what materials you will use.

  3. Variable • A variable is any factor that can affect the results of an experiment. • The variable is what changes.

  4. Example • 1. Label five equal-sized beakers as no salt, 5mL, 10mL, 15mL, and 20mL. • 2. Add 250mL water to each beaker. • 3. Measure out four different amounts of salt 5,10,15, and 20mL. • 4. Stir contents of each beaker to dissolve salt. • 5. Place each beaker on a hot plate. Heat until water boils. Use a thermometer to find the boiling point of the water in each beaker. • 6. Record measurements.

  5. List of the variables in the example experiment: • The size of the beakers • The volume of water used in each beaker • The method used to heat the water • The amount of salt added to each beaker.

  6. Two Types of Variables • Independent variable – is the factor that you change in order to find out what will happen. In the example experiment the independent variable is the amount of salt placed in each beaker. • Dependent variable- is a variable that can be affected by changes in the independent variable. The dependent variable in the example experiment is the boiling point of the water. You want to see if the boiling point changes depending on the amount of salt you add to the water.

  7. Control Group • The control group is a group used as a standard of comparison. Example: If you were studying the effects of a drug on mice, the control group would not get the drug. What is the control group in the example experiment? Answer: The beaker of water with NO salt.

  8. Experimental Group • The experimental group is the group that is exposed to changes in the independent variable. All other conditions are the same as those for the control group. • The experimental group in the drug testing would be the group of mice that received the drug. • What does the experimental group with the water and salt include? • The four beakers in which different amounts of salt have been added.

  9. Testing of Variables • Having a control and an experimental group in an experiment helps make sure that only one variable is tested at a time. • Testing only one variable makes it more likely that the results of the experiment are caused by that variable.

  10. Robin’s Egg Experiment • Observe robins nesting in a tree. • Observation: “Do robins eggs hatch faster at warmer temperatures?” • Hypothesis: Warmer temperatures decrease hatching time for robin’s eggs. • A good hypothesis states how changing the independent variable causes a change in the dependent variable.

  11. Robin’s Egg Experiment Independent variable : the temperature Dependent variable: The time it takes the eggs to hatch.

  12. Trial • Scientists usually repeat an experiment many times. Each time, they follow exactly the same procedure. • Each repetition of an experiment is called a trial. • Repeating experiments lets scientists check their results.

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