Inferences about a study

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Difficulty: Advanced
Estimated time: 35 minutes
Practice: Inferences question bank

What you’ll learn

Recognize a study question: a blank after groups are compared or a hypothesis is tested.
Name each group or condition and what it showed.
Cross out causes that both groups shared.
Apply a study's result to predict a new case.
Say what a study can't decide when its groups differ in two ways.
See how a study's design rules out another cause.

Prerequisites

Why this matters on the SAT

What the groups show, and what they can’t

A study text tells you what researchers did: compared two groups or two conditions, or put a hypothesis to a test. Then it stops at a blank and asks: Which choice most logically completes the text? About a third of inference questions report a study, nearly all of them in science. The clues are the groups: what each one showed, and what they had in common. So the question asks: what does the difference between the groups show, and what can't it show?

SAT example

Researchers studying motor learning randomly assigned experienced pianists to two practice schedules for an unfamiliar finger sequence. Each pianist completed 30 minutes of active practice: one group practiced continuously, while the other completed six five-minute practice periods separated by five-minute rests. Immediately after practice, the groups performed with similar speed and accuracy. When tested again 24 hours later without additional practice, the group that had taken rests performed more accurately, though not faster, than the continuous-practice group. Both groups also reported similar levels of concentration during active practice. These results suggest that _____

Which choice most logically completes the text?

A
rest periods improve both the speed and accuracy of a newly practiced sequence, but only after a night's sleep.
B
continuous practice is more useful than interrupted practice when musicians need accurate performance immediately.
C
the later accuracy difference likely arose because the interrupted-practice group concentrated more intensely during practice.
D
rest periods can improve delayed accuracy even when they yield no immediate performance advantage.

Solution to the example

Groups: pianists practiced nonstop or with rests. Right after, they matched; a day later, the rest group was more accurate, not faster.

Shared by both: how hard they said they concentrated.

Predict: rests helped accuracy a day later, with no edge right away.

Cross out: A adds speed, which the rests didn't give, and an untested only after a night's sleep. B gives nonstop practice an edge right after, when the groups matched.

Check: D fits: rests can improve delayed accuracy with no immediate performance advantage. The answer is D.

C tempts most: it credits concentration, shared by both, so it can't explain the gap.

The answer comes from the difference, not a cause shared by both

Study questions take the inference routine, with the groups in place of the clues and one new step:

Read what the blank needs: a conclusion, a prediction, a reason or a limit.
Name each group and what it showed, in a few words. A group can also be a condition, such as a later test or a warmer room.
Say what the difference shows, and what it can't: a cause shared by both groups can't explain it, and a second difference means the study can't decide.
Predict small: the smallest conclusion that fits every group.
Cross out choices that go too far, go past the text, turn it around, ignore the other explanation or name a cause already ruled out, then check the last choice against every group.

Step 3 is the new one. A study tries to keep its groups alike except for the thing it tests, so the answer comes from the difference between the groups.

When the difference shows up under one condition only, the answer keeps the condition: in the opening question, rests helped a day later, not right away. Where the groups match, all you learn is that the tested factor made no difference there, not that it can't work there.

Texts often say what was kept alike: randomly assigned means chance decided who went into each group, so the groups start out alike, and similar for both groups means that factor was matched. Here's a short study of our own:

The tomato trays

A science class grew two trays of tomato seedlings from the same seed packet. Both trays got the same soil and the same amount of water. One sat under a blue lamp, the other under a red lamp. After three weeks, the blue-lamp seedlings were shorter. This suggests that _____

Note: Groups: blue lamp shorter, red lamp taller
Note: Shared by both: seeds, soil and water
Note: Predict: the lamp color may have affected how tall they grew

Four ways to finish it:

Correct: Small enough: the lamp color may have affected how tall the seedlings grew.
Wrong: Shared by both: the soil kept the blue-lamp seedlings short.
Wrong: Too far: blue light makes every kind of plant grow shorter.
Wrong: Turned around: the red lamp kept the seedlings short.

The soil is a natural suspect, since soil feeds plants. But both trays had the same soil, so it can't explain why one came out shorter. A cause shared by both groups can't explain a difference between them. This trap tempts because it usually names a familiar cause. The other two fail the usual ways: one class's tomatoes can't speak for every kind of plant, and the red lamp kept nothing short, since its seedlings were the taller ones.

Check your understanding:

Can the quiet room explain the gap? Two classes took the same test in the same quiet room. One class had studied with flashcards, the other by rereading notes, and the flashcard class scored higher.

In this question, the nest boxes differ in sunlight. Name each group and what it showed, then cross out any cause shared by both.

Shaded nest boxes

Practice problem

Ecologist Priya Nair monitored artificial nesting boxes used by tree swallows. Some boxes received direct afternoon sunlight, while others were placed beneath shade cloth. On hot days, temperatures in the sunlit boxes frequently rose above levels considered safe for young birds, and adults abandoned 8 of the 20 nests in those boxes. The shaded boxes remained cooler, and adults abandoned only 1 of the 20 nests there. Food availability and predator activity were similar for both groups. These results suggest that ______

Which choice most logically completes the text?

A prediction applies the result to a new case

Some blanks ask what should happen next, after words like should likely, will likely or would most likely. A study's result, or the hypothesis it tests, works like a rule. Apply it to the new case, and predict no more than it covers. Here are the tomato trays again, with a hypothesis:

The tomato trays: a prediction

A science class grew two trays of tomato seedlings from the same seed packet. The blue-lamp seedlings came out shorter than the red-lamp ones, so the class thinks blue light slows stem growth. If so, adding a blue lamp over the red-lamp tray should most likely _____

Note: Hypothesis: blue light slows stem growth
Note: New case: the red-lamp tray gets blue light too
Note: Predict: its stems should grow more slowly

Correct: Small enough: slow how fast those seedlings grow taller.
Wrong: Too far: stop those seedlings from growing at all.
Wrong: Turned around: make those seedlings shoot up faster.
Wrong: Past the text: make those seedlings bear fruit sooner.

Slower isn't stopped, and nothing in the text is about fruit. A prediction keeps the result's direction, doesn't stretch it, and moves it to the new case.

Check your understanding:

Predict the blank. Seeds of one bean variety that were soaked overnight sprouted two days sooner than dry seeds of the same variety. If a gardener soaks a new batch of that variety overnight, the seeds will most likely _____

In this question, the blank asks for a prediction (should likely). Name each condition and what it showed, then apply the result to the new case, and no further.

Reminders on the screen

Practice problem

In an experiment on sustained attention, neuroscientists monitored participants’ brain activity while the participants performed a long, repetitive task that required them to quickly respond to certain symbols on a screen. When participants’ activity increased in the brain’s default mode network—a set of regions associated with internally focused thought such as daydreaming—they were more likely to miss the target symbols. However, when the researchers occasionally displayed a brief on-screen reminder about the task goal, default mode network activity decreased and participants missed fewer targets. Consequently, brief reminders should likely help people to ______

Which choice most logically completes the text?

When the groups differ in two ways, the study can’t decide

Sometimes the groups differ in more than the thing being tested. That second difference is another cause: when either difference could produce the result, the study can't decide which one did. So expect a limit, what the study can't show, even when the lead-in says only “therefore”. Here are the tomato trays with a second difference:

The tomato trays: a second difference

A science class grew two trays of tomato seedlings from the same seed packet. The blue-lamp tray sat by a cold window, and the red-lamp tray by the heater. Cooler air can also slow seedlings' growth. After three weeks, the blue-lamp seedlings were shorter. On its own, this result _____

Note: Blue lamp: by a cold window, shorter seedlings
Note: Red lamp: by the heater, taller seedlings
Note: Second difference: the cold can slow growth too
Note: Predict: this result alone can't show the lamp color did it

Correct: A limit: doesn't show whether the lamp color made the difference.
Wrong: Too far: proves that the cold, not the light, kept them short.
Wrong: Ignores the other explanation: shows that blue light keeps seedlings short.

The cold doesn't rule out the light. It only means the result could have come about either way. When both explanations predict the result, the answer says the study can't decide, not which one is right.

Check your understanding:

Can this study show the drink helped? Runners who drank a new sports drink ran the same 5K course faster than runners who drank water. The drink group ran on a cool morning, the water group on a hot afternoon, and heat slows runners down.

In this question, the lead-in says only “therefore”, but the two sets of slices differ in two ways. Find the second difference, then predict what the study can't show.

Coated pear slices

Practice problem

Food scientists tested whether a new edible coating slows the browning of sliced pears. They placed coated slices in one set of containers and uncoated slices in another set of containers, then measured how dark the slices became after 6 hours. After analyzing the data, they discovered that the containers holding the coated slices had been stored in a refrigerator, while the containers holding the uncoated slices had been left on a lab bench. Because cooler temperatures slow enzymatic browning, the results of the study, therefore, ______

Which choice most logically completes the text?

A comparison can rule out the other cause

To settle a second difference, a study gives the other cause to a group without the one being tested. If that group doesn't show the result, the other cause is ruled out. Here the class runs the trays again:

The tomato trays: a rerun

A science class grew two trays of tomato seedlings from the same seed packet. This time both sat by a cold window, one under a blue lamp and one under a red lamp. After three weeks, the blue-lamp seedlings were still shorter than the red-lamp ones. This suggests that _____

Note: Blue lamp, cold: shorter
Note: Red lamp, cold: taller, so the cold can't explain the gap
Note: Predict: the lamp color, not the cold, probably made the difference

Correct: Fits both trays: the lamp color, not the cold, probably made the difference.
Wrong: Already ruled out: the cold window kept the blue-lamp seedlings short.

The red-lamp tray had the cold without the blue light, and its seedlings were still the taller ones. With both trays by the window, the cold is shared by both, so it can't be the reason.

Details of a design can rule causes out too. In a test of whether dogs learn which cup color hides a treat:

The design	Rules out	Example
fresh, unused test materials	anything left over from before	clean cups, so no treat smell is left
places shuffled before the test	a memory of where the reward was	cups moved, so the old spot means nothing
roles swapped between groups	one option just being better on its own	half the dogs learn blue, half learn yellow

The tempting wrong choices are often the very explanations the design already ruled out.

Check your understanding:

Which explanation does this run rule out? In another run, the blue lamp sits by the heater and the red lamp by the cold window. The blue-lamp seedlings still come out shorter.

In this question, each detail of the design rules out one explanation. Match each wrong choice to the detail that rules it out.

Rings or stripes

Practice problem

Some flowers have ultraviolet patterns that are difficult for humans to see. To test whether hawkmoths can learn to use such patterns when searching for food, researchers created artificial flowers with either a ring pattern or a striped pattern. For half the moths, flowers with rings contained sugar solution; for the other half, flowers with stripes contained the solution. The researchers then presented each moth with a new set of empty flowers and rearranged the flowers’ locations. In both groups, the moths approached flowers displaying the pattern that had previously been paired with sugar more often than they approached the other flowers. These results suggest that _____

Which choice most logically completes the text?

Example: what a first trial can’t show

In this question, a first trial mixes two causes, and a follow-up pulls them apart. Name each group, then ask what the first result can't show on its own.

Worked example

Biologists studying island wrens tested whether alarm calls reflect an innate recognition of invasive rats. In an initial field trial, wild adult wrens from rat-colonized sites called more at rat-shaped models pulled through leaf litter than at harmless lizard-shaped models displayed motionless. A follow-up trial varied shape and movement independently. Captive juveniles that had never encountered rats called more at moving than at stationary models but showed no preference for either shape. Wild adults also called more at moving models, yet when both models were stationary, the adults called more at the rat-shaped one. The researchers note that experience with rats can alter adult responses. Taken together, these findings most strongly suggest that ______

Which choice most logically completes the text?

A
the adults’ response to stationary rat-shaped models is best explained by an inherited visual-recognition mechanism that becomes stronger with age.
B
the difference observed in the initial field trial was caused by model movement rather than by model shape.
C
the initial field result cannot by itself demonstrate innate recognition of rats, since movement and prior experience can each influence alarm calling.
D
experience with rats caused adults to replace a general sensitivity to movement with a shape-specific response.

Step 1

Read what the blank needs

The biologists tested whether alarm calls reflect an innate recognition of invasive rats. Innate means inborn: there from the start, without any experience of rats. The blank follows Taken together, these findings most strongly suggest that, but the text also says experience can change adults' calls: a second explanation. So expect a limit: what the first trial can't show.

Step 2

Name each group and what it showed

The first trial compared rat-shaped models pulled through leaf litter with lizard models that stood still. The follow-up varied shape and movement separately: young birds that had never met rats showed no preference for either shape, and with both models still, the adults called more at the rat-shaped one.

Note: First trial: adults from rat sites; moving rats, still lizards
Note: Young birds: more calls at movement, no shape preference
Note: Adults: more calls at movement, and at the rat shape when both stood still
Note: Experience: can change how adults respond

Step 3

Say what the first trial can’t show

In the first trial, the rat models differed from the lizard models in two ways: shape and movement. Movement is a second difference, and the young birds show it raises calls on its own. The adults in the first trial also came from rat-colonized sites, and the researchers add that experience with rats can alter adult responses. So even the adults' reaction to the rat shape may be learned, not inborn.

Predict: the first trial alone can't show an inborn recognition of rats, because movement or experience could explain it.

Step 4

Cross out what goes too far

The right answer has to fit both groups in the follow-up:

The answer must fit	A	B	C	D
1. Young birds: movement, not shape	✗ no sign	✓	✓	✓
2. Adults: movement and shape	✓	✗ shape too	✓	✗ movement stays

B is the most tempting, because it follows the young birds: movement mattered, shape didn't. But the adults did respond to shape when both models were still, so rather than by model shape goes too far. A calls the adults' response best explained by an inherited visual-recognition mechanism, which goes too far: the young birds, which had never met rats, showed no shape preference, and experience could explain the adults' response. D has adults replace their sensitivity to movement, but they still called more at moving models. A, B and D go.

Step 5

Check the last choice

C has two halves. The initial field result cannot by itself demonstrate innate recognition of rats is the limit, and since movement and prior experience can each influence alarm calling gives the two reasons: the young birds' calls at moving models and the researchers' note on experience. Both halves hold. The answer is C.

Example: a study that solves a puzzle

In this question, a study takes on a puzzle: seedlings gather around adult plants, yet the adults lower their survival. Name each group, then find the one idea that fits both facts.

Worked example

In salt marshes, cordgrass seedlings often occur in rings around mature plants, a pattern generally interpreted as evidence that the adults facilitate seedling establishment. Ecologists released equal numbers of marked cordgrass seeds beside live adult clumps, beside resin replicas matching the clumps' stem arrangement, and on adjacent bare sediment. After several high tides, the live clumps and replicas had trapped similarly high numbers of seeds, whereas the bare areas retained few. Yet among the trapped seeds that germinated, survival through midsummer was lower beside live clumps than beside replicas; measurements showed comparable soil moisture and salinity but substantially less light beside the live clumps. The ecologists therefore hypothesize that _____

Which choice most logically completes the text?

A
rings of seedlings provide reliable evidence that adult plants increase seedlings' overall chances of reaching maturity, even when some seedlings are shaded.
B
rings of seedlings can arise through passive capture by adults' physical structure even though living adults reduce the seedlings' later survival.
C
rings of seedlings are caused mainly by seeds germinating more readily in the lower light available beside living adults.
D
rings of seedlings would be equally pronounced on bare sediment if its soil moisture and salinity matched those beside adult plants.

Step 1

Read what the blank needs

The text opens with the usual reading of the rings: the adults facilitate seedling establishment, that is, adult plants help seedlings take hold. Then “Yet” brings a result that pulls the other way. The blank asks what the ecologists hypothesize, so it needs one explanation that fits every result. As with any puzzle, keep both facts, and look for the idea that fits them together.

Step 2

Name each group and what it showed

Seeds went to three places. Live clumps and resin copies trapped similarly high numbers of seeds, while the bare areas retained few. But survival through midsummer was lower beside live clumps than beside replicas.

Note: Live clumps: many seeds; fewer seedlings survive
Note: Resin copies: as many seeds; more seedlings survive
Note: Bare mud: few seeds

Step 3

Say what each difference shows

Two pairs of groups each differ in one way. Copies against bare mud: only the copies had the plants' shape, and they caught the seeds, so the shape traps seeds. Live clumps against copies: same shape, and comparable soil moisture and salinity, shared by both, so they can't explain the gap in survival. What differed was substantially less light beside the live clumps.

Predict: the adults' shape traps seeds into rings, but living adults lower the seedlings' survival, probably by shading them.

Step 4

Cross out what goes too far or past the text

The answer must fit	A	B	C	D
1. The shape traps the seeds	✗ ignores it	✓	✗ germination	✗ moisture
2. Living adults lower survival	✗ says help	✓	silent	silent

A is the most tempting, because it's the usual reading the text opens with. But the replicas, which can't help anything grow, trapped just as many seeds, so rings form without any help from living adults, and seedlings survived less well beside live adults than beside replicas. So the rings aren't reliable evidence that adults help: A goes too far. C credits seeds sprouting more readily in low light, but nothing compares how many seeds sprouted: it goes past the text. D goes past the text: nothing ties the rings to moisture or salt, and bare mud kept few seeds because it had no shape to catch them. A, C and D go.

Step 5

Check the last choice

B has two halves, one for each fact. Rings of seedlings can arise through passive capture by adults' physical structure is the trapping: the plants catch seeds just by standing in the way, as the lifeless copies did. Even though living adults reduce the seedlings' later survival is the survival result. Both halves hold, and can arise keeps it small. The answer is B.

Practice problems

These questions mix the cases, so on each one, name each group and what it showed, and say what the difference shows before you read the choices.

Mussels in the tank

Practice problem

Researchers placed equal volumes of pond water in two tanks. One tank contained native freshwater mussels, while the other did not. After 24 hours, the water in the tank with mussels contained fewer suspended particles and was clearer. The researchers repeated the trial and obtained similar results.

The results suggest that ______

Which choice most logically completes the text?

Orchids after logging

Practice problem

Field botanists studying a rare forest orchid have shown that its seeds germinate only in the presence of a specific soil fungus. In recently logged plots, they found far fewer seedlings than in nearby unlogged plots, even though adult orchid numbers were similar in both areas. These observations most strongly suggest that the logging likely ____

Which choice most logically completes the text?

An enzyme and acidity

Practice problem

A team of environmental microbiologists studied how the bacterium Pseudomonas putida breaks down a harmful industrial byproduct, compound X, in water. The team deleted the gene that produces AcdA, an enzyme suspected to help degrade compound X, and compared the modified bacteria with unmodified P. putida. In acidic water (pH 5), both the unmodified and modified bacteria reduced compound X to similarly low levels after 24 hours. In near-neutral water (pH 7), however, the unmodified bacteria reduced compound X substantially while the modified bacteria did not, which suggests that ______

Which choice most logically completes the text?

Finish the lesson

7 practice examples left

Finish the remaining questions correctly to complete this lesson.

Quick recap

Shared by both: the answer comes from the difference between the groups, and a cause they had in common can't explain it.
A prediction moves the result to a new case, in the same direction and no further.
Another cause: when a second difference could also explain the result, the study can't decide; a choice that picks a side goes too far, and one that ignores the other explanation misses the limit.
Already ruled out: a group with the other cause but not the result, or a detail such as fresh materials, shuffled places or swapped roles, rules that cause out.
Every question: read what the blank needs, name each group and what it showed, say what the difference shows and can't, predict small, then cross out and check the last choice.

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