what is a controlled experiment and why
would you want to do one maybe you have
an idea that you think might explain a
situation this is called a scientific
hypothesis how could you find out if
your hypothesis is correct
well you'd set up a controlled
experiment in which you control or keep
constant all the factors known as
variables except for the one you want to
test let's design a controlled
experiment to test a fertilizer which
claims it makes plants grow bigger
lusher and perhaps produce more flowers
fruit or vegetables if you wanted to see
if the fertilizer works how would you
set up a controlled experiment to test
this claim first you would get two
plants of the same species let's call
them plant a and plant B everything
about the plants should be exactly the
same including their size health and age
next you would put each plant in
identical pots with the same amount of
the same kind of dirt or soil you would
water them both the same amount at the
same times you would also put the plants
next to each other in the same place
such as a windowsill so that they're
both exposed to the same amount of
sunlight and kept at the same
temperature it's important that
everything is the same because the
purpose of your experiment is to find
out whether or not the fertilizer works
so what would be different
in this experiment the only difference
is that only plant a would get the
fertilizer now remember your hypothesis
is the plant a which is getting
fertilizer will grow bigger compared to
Plan B which isn't getting any
fertilizer how would you know whether
your hypothesis is correct you'd know
because you'd regularly measure the
plant during the course of the
experiment for example once a week for a
period of three months you would record
these measurements throughout the
experiment these measurements are your
data at the end of the experiment you
would look at your data and compare the
measurements of plant a witch got
fertilizer to plant B which didn't get
fertilizer as you can see plant a did
grow bigger than plant B so it appears
that the results of this controlled
experiment support your hypothesis so
let's recap the elements of experimental
design what were you testing you were
testing to see whether or not fertilizer
promotes plant growth what was your
hypothesis the hypothesis was that the
plant that got fertilizer would get
bigger than the plant that didn't get
fertilizer what were you measuring you
measured the growth of both plants how
do you know if the results of the
experiment support your hypothesis well
if your hypothesis is true you would
have seen that the plant that got the
fertilizer actually did get bigger than
the plant that didn't get any fertilizer
the variable you were testing in this
case the fertilizer is called the
independent variable and the thing you
were observing measuring and expecting
to change because of that independent
variable was plant growth in this
experiment plant growth is the dependent
variable we'll go over independent and
dependent variables in more detail in
another video