Read a biology experiment: variables, controls, repeated trials

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

1. Read this first

   Two data sets on plant food (example data)

   data set	plants per group	unfed mean (cm)	fed mean (cm)
   school archive, five years	100	15.1	16.8
   own test	6	15.6	15.9

2. 1.

   Ari compares an archive with a small test. How should the two data sets be read together?

   • Average the two gaps to get 1.0 cm, and report that as the effect size
   • They agree in direction, and the 100 plants in the archive make its 1.7 cm gap firmer
   • The small test overturns the archive, since it found almost no gap between its groups
   • The archive is wrong, because it is older than the new test is, and so out of date

3. Two plant food studies (example data)

   study	unfed mean (cm)	fed mean (cm)
   tomato	10.0	12.0
   bean	20.0	22.4

4. 2.

   Bo compares the two studies. Which gives the larger effect in relative terms?

   • The bean study, since its 2.4 cm gap is larger than the tomato gap of 2.0 cm
   • Both are the same, since each gap is about 2 cm
   • Tomato: +20 percent against +12 percent, though the bean gap is larger in cm
   • The bean study, since its fed mean of 22.4 cm is the larger of the two

5. Result (example data)

   The plants given plant food sit by the window, and they are taller, 18 cm against 15 cm. Explanation 1: the plant food helped. Explanation 2: the window light helped.

6. 3.

   Jun reads the result. Which extra test best separates the two explanations?

   • Add a third group with extra plant food by the window
   • Put some fed and some unfed plants both by the window and away from it
   • Repeat the test with the same positions and many more plants in each
   • Measure the plants by the window more often, to be sure of their heights

7. Four trays (example data)

   tray	treatment	mean height (cm)
   A	plant food	18
   B	plant food	15
   C	no plant food	14
   D	no plant food	12

8. 4.

   Kira had two trays for each treatment. What do the means show about a tray effect?

   • Fed plants are clearly taller on average: 16.5 cm against 13 cm
   • Tray effects do not matter, since each tray held many plants to average
   • Trays given the same treatment differ by 2 to 3 cm, so plant food is unclear
   • Trays A and D show the effect clearly: 18 cm against 12 cm, a 6 cm gap

9. Six lakes (model data)

   lake	phosphorus (µg per L)	algae, as chlorophyll (µg per L)
   1	10	3
   2	20	6
   3	35	9
   4	50	14
   5	70	20
   6	90	27

10. 5.

    Luz sees both numbers rise together across lakes. Does this show phosphorus causes more algae?

    • No: six lakes are too few to show any pattern in the data
    • Yes: the algae rise as phosphorus rises in each of the six lakes studied
    • Yes: the pattern is almost a straight line, so one must cause the other one
    • No: sunlight or warmth could drive both; adding phosphorus would test it
11. 6.

    The first 10 minutes gave 3 mL per minute. Milo predicted 90 mL at 30 minutes from the first 10 minutes. The reading was 62 mL. What does this show?

    • The 62 mL reading must be a measuring error, since the model said 90 mL
    • The model is right, since gas is still rising at 30 minutes, just as it predicted
    • The straight-line model fits the early readings, but the rate slowed later
    • The yeast stopped making gas at minute 10, when the rate was highest
12. 7.

    Noah uses the model for predictions. Which prediction is least trustworthy?

    • Day 60: 94 cm, far outside the days from 2 to 6
    • Day 5: 11.5 cm
    • Day 3: 8.5 cm
    • No prediction is doubtful, since a model that fits days 2 to 6 fits the later days too
13. 8.

    Seedling A, height = 4 + 1.5 × day; seedling B, height = 1 + 2.5 × day. Pia wants to know when seedling B is as tall as seedling A. On which day are they the same height?

    • Day 2, since 4 − 1 = 3 and 3 ÷ 1.5 = 2
    • Day 3, when both are 8.5 cm tall
    • Day 1.2, since 3 ÷ 2.5 = 1.2
    • At no point, since seedling A starts taller than seedling B
14. 9.

    Working explanation: warmer water speeds up the yeast. Tala wants new data to test the explanation. Which new result would most challenge it?

    • Bubbles forming faster at 50 °C, which would fit the pattern
    • Three bottles at 40 °C giving 21, 22 and 23 mL, close to the mean
    • More gas at 35 °C than at 30 °C, which would fit the pattern in the table
    • Less gas at 50 °C than at 40 °C, which would show that heat can harm yeast

15. Two yeast studies (example data)

    study	bottles per temperature	yeast batches	how gas was measured
    A	4	1	syringe volume
    B	1	3	bubbles counted by eye

16. 10.

    Both studies found more gas at 40 °C. Zee compares them. Which statement is best?

    • They are equally strong, since both found more gas at the same temperature
    • B is stronger, since counting bubbles is a more direct measure of gas
    • B is stronger, since it used three yeast batches against one for A
    • A is stronger: replicates, one yeast batch and a measured volume cut chance