WHAT MAKES A GOOD EXPERIMENT?
An experiment is a controlled procedure designed to test a
hypothesis. There are several
parts of any good scientific experiment.
A good experiment is one that
is:
Interesting
Important
Organized
Valid
Repeatable
HYPOTHESIS: How ____________ affects _______________.
Cause (IV) and Effect
(DV).
More than a
prediction about the outcome of an experiment, a hypothesis provides an
explanation for why phenomena occur. A valid hypothesis includes the property
that is changed (the cause or the independent variable) and the property that
is measured (the effect or dependent variable) which may change as a
result. In a good experiment,
there is only one cause for the effect.
Example:
Increasing the amount of light (cause) increases a plant’s height
(effect). (Light affects height)
Both of these
properties can be measured. When
they are graphed, the independent variable is placed on the bottom (x) axis,
and the dependent variable is placed on the side (y) axis.
INDEPENDENT VARIABLE
(IV): This is the factor, or variable, that you change. This
is the physical property that you have direct control over to change. It should
be the ONLY difference between the two groups for it to be a good
experiment. It is the CAUSE
property mentioned in the hypothesis.
Example:
the amount of light.
DEPENDENT VARIABLE
(DV): This is the factor, or property, that you measure for,
or the result. It could be
different between the groups, or it could be the same. You don't know the value of this
variable until the end of the experiment.
This is the EFFECT property mentioned in the hypothesis.
Example:
how high the plant grows, the height
CONSTANTS/CONTROLLED
EXPERIMENT: All other properties and factors should be the SAME in
all groups, or they should be CONTROLLED.
Example: the amount of
food, the amount of air, the type of plant, are all kept the same.
A CONTROL GROUP: The
group that is used as the basis for comparison. It could be: the BEFORE part of a before and after
experiment (mixing two chemicals to see a color change, the control group is
the set up before they were mixed).
It could be: the "normal,” or it could be the group in which the
value of the independent variable is zero.
Example: a plant set in a
room with no light bulb.
EXPERIMENTAL GROUP
(S): The experimental group differs from the control group
in just ONE factor or variable.
This is the group that is usually mentioned in the prediction. It can also be the "after"
part of a before and after experiment.
It is the actual physical set of objects that you have changed or are
doing something to. A good
experiment usually has at least two or three experimental groups, or data
points.
Example: a plant set in a
room with twenty light bulbs.
CONCLUSION: You organize your
results and data and summarize patterns in the data. The
conclusion statement should indicate whether the evidence supports your initial
expectations and describes
what you found out about the hypothesis and prediction.
The conclusion should also discuss the validity and reliability of your
experimental results.
EXAMPLE of WHAT MAKES A GOOD
EXPERIMENT?
Amount of Light (Cause, IV) affects the Height
(Effect, DV) of a plant.
A good experiment, besides having careful observations, and using instruments, will always have a control group to compare to. It is also very important to have groups in which only ONE property or variable is changed at a time, so that you can be sure that the property is the cause of whatever effect you are measuring.
Amount of Light Average Height of plant after 24 hours
1 foot candle (19 cm, 20cm, 21 cm) 20 cm
2 foot candle (30 cm, 25 cm, 29 cm) 28 cm
10 foot candles (110 cm, 113 cm, 119 cm) 114 cm
To determine the relationship, a student could find a "best fit" line or curve.
Y= 2X + 10 , so with NO light, the plant would be at 10 cm (Control Group= Y Intercept)
If there is more than one independent variable, bad experiment = Not a Function!