GED Science
Scientific Method & Experiment Design
An experiment is a fair test: change exactly ONE thing, measure ONE result, hold everything else identical, and include a comparison group that gets no treatment. Most questions hand you a flawed experiment and ask what's wrong — the answer is almost always a second thing changing.
The words first
- hypothesis
- a testable if-then prediction, not just a guess
- independent variable
- the ONE thing you deliberately change
- dependent variable
- the result you measure
- control group
- the comparison group that gets no treatment
- constants
- everything held identical between groups
The method
THE FAIR-TEST BLUEPRINT
question: Does fertilizer X make tomatoes grow bigger?
hypothesis: IF plants get X, THEN they'll yield heavier fruit.
change ONE: fertilizer X (given / not given) ← independent
measure ONE: fruit weight ← dependent
hold same: sunlight, water, soil, pot, plant type ← constants
compare: an untreated group ← control
SPOT-THE-FLAW CHECKLIST (the test's favorite)
□ did TWO things change at once? → can't tell which one acted
□ no control group? → nothing to compare against
□ tiny sample (2 plants)? → luck can explain the result
□ groups treated differently → hidden second variable
(sunny window vs dark shelf)
results ≠ proof: one experiment SUPPORTS a hypothesis;
repetition by others is what builds confidence
Worked all the way through
The problem
A student gives fertilizer to tomato plants on a sunny windowsill and no fertilizer to plants in the garage, then concludes fertilizer works because the windowsill plants grew more. What's wrong?
- Step 1 — List everything that differed between the groups.Fertilizer, yes — but ALSO sunlight, temperature, and location.
- Step 2 — Apply the one-change rule.Two or more things changed at once, so the result cannot be pinned on fertilizer. The sun alone could explain everything.
- Step 3 — Name the fix.Put both groups in the same spot with the same light, water, and pots, differing in fertilizer ONLY.
On the board
group A: fertilizer + sun
group B: nothing + garage
two variables changed
fertilizer AND light
conclusion is unearned
sun could be the cause
fix: same place, same care, fertilizer only
Answer
Light and location changed along with the fertilizer — move both groups to identical conditions
A second one, in a different form
In a headache-pill trial, one group gets the pill and the control group gets a sugar pill that looks identical. Why bother with the fake pill?
believing you're treated can itself help
the placebo effect
identical-looking pill hides who got what
any extra improvement = the medicine
belief effect subtracted out
Answer
So both groups believe equally — real improvement beyond the sugar pill is the drug's actual effect
The usual mistake
Mixing up independent and dependent. The dependent variable is the RESULT.
Check yourself
For any experiment question, underline every difference between the groups. Finding exactly one difference means a fair test; finding two means you've found the flaw the question wants.
Practice it
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