Scientific Method and Measurement

This topic opens General Science and it is the most answerable one in it, because it is vocabulary and logic rather than content. Get these terms exactly right and you bank a question or two before any actual science is involved.

The method

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1. Observation. Something is noticed.
2. Question. What accounts for it.
3. Hypothesis. A testable, falsifiable proposed explanation. "If I do X, then Y will happen."
4. Experiment. A test that isolates one variable.
5. Analysis. What the data show.
6. Conclusion. Whether the hypothesis is supported, and it is supported or not supported, never proved.

A hypothesis must be testable and falsifiable. "Plants grow faster with more light" is a hypothesis. "Plants prefer sunlight" is not, because nothing would count as evidence against it.

Results are shared at the end, by publishing them so other scientists can check the work and repeat it. That checking is called peer review, and it is why a result nobody else can reproduce does not count for much.

Two kinds of observation

Qualitative observations describe qualities: color, smell, texture, "the solution turned cloudy." Quantitative observations are measurements with numbers: 24 centimeters, 3.5 grams, 40 degrees. The word is the clue - quantity means an amount.

The variables

The most tested vocabulary in the topic.

The dependent variable depends on the independent one. That sentence is the mnemonic and it is exact.

Controlled variables and the control group are different things, and questions exploit the similar names. Controlled variables are the conditions you keep constant. The control group is a set of subjects that gets no treatment.

One variable at a time

If two things change at once, the experiment cannot tell you which caused the result.

A gardener gives one plant more fertilizer and moves it to a sunnier window. It grows faster.

This experiment establishes nothing. The fertilizer might have done it, the light might have done it, or the two together.

A question describing such a setup is asking you to identify the flaw, and the answer is always some version of "more than one variable changed".

Sample size and repetition

Two other ways an experiment fails, both asked directly.

Too small a sample. One plant is not evidence, because plants vary. A larger group averages out the random variation that has nothing to do with the treatment.

No repetition. A result that happens once might be chance. Repeating it is what separates a finding from an accident. A result that comes out the same when repeated is called reliable, and one other scientists can get too is reproducible. Trials that give very different results each time have high variability, which is a sign the experiment is not controlling something.

Error and bias

Two kinds of error, and the difference is the question.

A scale that always reads 2 grams heavy has a systematic error, and averaging more readings will not help, because every one is wrong the same way.

Bias is a systematic error that comes from people.

• Experimenter bias: the researcher, without meaning to, nudges results toward what they expect.
• Subject bias: the subjects behave differently because they know what is being tested, or when.
• Confirmation bias: noticing the evidence that fits and discounting what does not.

The fixes are blinding and randomness. In a blind study the subjects do not know whether they got the real treatment or a placebo (a fake with no active effect). In a double-blind study the researchers do not know either, which is what removes experimenter bias as well. Random sampling - choosing subjects by chance - keeps the groups from being skewed by how they were picked.

Law against theory

A theory does not graduate into a law. They are different kinds of statement, not different ranks, and a question implying that a theory becomes a law once proven is testing exactly this.

SI units

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The measurement half of the topic. These are the base units worth knowing.

And the derived units that turn up:

Mass is measured in kilograms and weight in newtons, because weight is a force. That distinction gets its own question in the Newton's laws lesson and it starts here.

The prefixes

And one more that is asked on its own: deci- means one tenth, so a decimeter is a tenth of a meter.

So a kilometer is 1,000 meters and a millimeter is a thousandth of a meter. Metric conversion is moving a decimal point, which is the whole reason the system exists.

Two anchors: one milliliter of water is one cubic centimeter and has a mass of one gram; one liter of water has a mass of one kilogram.

The tools

A balance measures mass, not weight, which is why it reads the same on the Moon. Read a graduated cylinder at eye level, at the bottom of the curved surface.

Accuracy and precision

Different words, and the test separates them.

Accuracy is closeness to the true value. Precision is how closely repeated measurements agree with each other.

A scale reading 2.00 kg every time for a 1.00 kg mass is precise but not accurate. A scale giving 0.9, 1.1 and 1.0 for the same mass is accurate on average but not precise.

A measurement can be either without the other, which is the point of asking.

Significant figures

The digits in a measurement that actually carry information. Three rules cover every question:

• Every nonzero digit counts. 45.67 has four.
• Zeros between nonzero digits count. 405 has three.
• Leading zeros never count; trailing zeros after a decimal point do. 0.00420 has three (4, 2 and the final 0). 5.20 has three.

The logic behind the last rule: 5.20 says the measurement was good to the hundredth, while 5.2 only claims the tenth. The extra zero is information.

Tables and graphs

Match the graph to what the data is doing.

The independent variable goes on the x-axis, the horizontal one, and the dependent variable on the y-axis.

Correlation describes how two variables move together.

• Positive correlation: both rise together. More sunlight, taller plants.
• Negative correlation: one rises as the other falls. More practice, fewer errors.
• Strength: a correlation near +1 or -1 is strong; near 0 there is no relationship.

Correlation is not causation. Ice cream sales and drownings rise together because both rise in summer, not because one causes the other. Only a controlled experiment can show cause.

Standard deviation measures how spread out the data are around the average. A small one means the values cluster tightly.

What you can skip

The bank's 139 questions on this topic never ask about:

• The less common metric prefixes. Mega-, hecto- and deka- appear in none of them. Know kilo-, centi-, milli-, micro-, nano- and deci-.
• Lab glassware beyond the graduated cylinder. No question asks about a beaker or a pipette.
• Statistical testing. P-values and significance tests do not appear. Standard deviation gets one question, and "the spread of the data around the average" is the whole answer.
• Naming famous experiments. The questions describe an experiment and ask you to reason about it. Knowing who did what, when, is never the point.

Where people lose points

Swapping the independent and dependent variables.

Confusing controlled variables with the control group.

Calling a theory a guess.

Saying an experiment proved a hypothesis. Evidence supports; it does not prove.

Mixing up accuracy and precision.

Giving weight in kilograms. Kilograms are mass.

Treating random and systematic error as the same. Averaging fixes random error and does nothing for systematic error.

Counting leading zeros as significant. 0.00420 has three significant figures, not six.

Reading correlation as cause.

Work one in under a minute

A researcher tests whether a coating reduces corrosion. Twenty steel plates are coated and twenty are left bare. All forty are placed in the same salt spray chamber for thirty days, then weighed.

What is the dependent variable?

The researcher changed the coating, so that is independent. What is measured afterward is the mass lost to corrosion, so mass loss is the dependent variable.

The twenty bare plates are the control group. The chamber, the duration and the salt concentration are controlled variables.

Where this leads

Every experimental question elsewhere on this subtest uses this vocabulary, and the SI units carry into physics, chemistry and Electronics Information.