Read a biology experiment: variables, controls, repeated trials
Grade 9 Science Worksheet
Circle the best answer.
Read this first
Gas from yeast at 30 °C (example data)
time (min) gas (mL) 0 0 2 6 4 13 6 18 8 26 10 30 Ari uses the mean rate from the first to the last reading to predict gas at 7 minutes. What is the prediction?
- 2.3 mL
- 21 mL
- 26 mL, the 8 minute reading
- 18 mL, the 6 minute reading
Esme draws a best-fit line through the origin with a slope of about 3 mL per minute. What does the slope mean?
- The yeast needs 3 minutes before it starts making gas
- The bottle held 3 mL of gas at the start
- Each minute, about 3 more mL of gas is made
- The yeast made 3 mL of gas in total
Plan (example data)
a student has 40 bean seedlings of different heights. The 20 tallest go in the plant-food group and the 20 shortest in the control group. All are measured after 14 days.
Jun reviews the plan. Which change fixes its main problem?
- Assign the seedlings to the two groups at random
- Use just the 20 tallest seedlings to make the test stronger
- Give both groups the same light
- Measure the plant-food group more often
Plan (example data)
The student who set up the groups measures every seedling and writes down its height, knowing which group it is in.
Kira reads the plan. Which change best reduces measurement bias?
- Switch to a ruler with finer marks
- Have a helper who does not know the groups do the measuring
- Add 20 more seedlings to each group
- Measure each seedling twice and keep the larger of the two values
Plan (example data)
10 plants with plant food sit in tray A, by the window. 10 plants without plant food sit in tray B, in the middle of the room.
Luz reads the plan. Which change best separates plant food from tray effects?
- Use several trays for each treatment, in mixed positions
- Measure just the plants in tray A
- Put 20 plants in each tray instead of 10 plants in each tray
- Water tray B more often
a gas syringe is read at a slant, so every reading comes out 2 mL too high. Milo repeats the test 10 times with the slanted syringe. What does repeating do for this error?
- Nothing: every reading is off the same way, so the mean is off too
- It removes the error, because more trials are better
- It doubles the error with each added trial
- It averages out over the 10 trials, so the final mean will be right again
a balance reads 0.3 g with nothing on it. Noah weighs seedlings on the balance. How should the readings be treated?
- Ignore it, since a small error cancels out
- Average many readings, which will remove the 0.3 g from the final result
- Subtract 0.3 g from each reading, since the error is the same each time
- Discard the balance, since 0.3 g makes the results invalid
Example data
a 5.0 g test mass is weighed four times on a balance. The readings are 3.1, 3.1, 3.2 and 3.1 g.
Pia reads the four readings. Which description fits them?
- Both accurate and precise: the readings agree with each other
- Precise but not accurate: the readings agree but sit far from 5.0 g
- Neither: the readings differ from each other a lot
- Accurate but not precise: the readings sit near 5.0 g
Seedling height after 14 days, 12 plants per group (example data)
group mean (cm) range (cm) control 15.0 12 to 18 plant food 17.0 14 to 20 Ren reads the group summaries. Which statement do the data best support?
- Plant food has no effect, since the ranges are not the same
- Plant food adds 2 cm to each seedling
- Plant food works, since the fed mean is higher
- The 2 cm gap may be chance: the ranges overlap from 14 to 18 cm
Tala reads these group summaries. Which statement do the data best support?
- No conclusion is possible, since means alone cannot show a cause
- The ranges do not overlap, so the 5.5 cm gap is likely real here
- The 12 cm and 21 cm extremes show the effect varies too much
- Plant food will add 5.5 cm to any plant