International A Level Biology / Practical Skills in Biology II
Recording and analysing data: tables, logs and tests revision notes
Edexcel International A Level Biology (YBI11), chapter 26: Practical Skills in Biology II, specification point U6.2, U6.3, A.2.5. Free lesson.
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Record raw measurements in table headings, check that values and units make sense, and choose a graph and test that fit the data. Finish statistical conclusions in biological terms.
Put raw data in the table headings
Raw data is measured directly with an instrument or counted. Put it in the table headings with its unit. Rates, percentages and means are calculated values, so add them in extra columns after the repeats. Respirometer: distance moved by the drop in 10 minutes / mm, not rate or RQ. Pondweed: length of the gas bubble in 5 minutes / mm, not rate of photosynthesis. Methylene blue: time for the blue to disappear / s. Brine shrimp: number of larvae hatched, not percentage hatched.
Check units, decimal places and rounding
Ask whether each value is realistic in its stated unit. Use the same number of decimal places for comparable means, and round to the requested significant figures rather than chopping digits. Pollen tubes of 450 and 620 mm would be over half a metre long. The measurements should be in µm. Write 36 as 36.0 when comparing it with a mean of 31.6. To round 2.37 to 2 significant figures, use the next digit, 7, and round up to 2.4. Write units clearly, for example dm³ min⁻¹ rather than dm³/min.
Use a log scale for a wide range of values
A logarithm is the power of 10 that gives a number. When values span several powers of ten, an ordinary axis can squash the smaller values near the bottom. A log scale lets them fit across the graph. Yeast counts rise from 2.0 × 10⁵ to 8.0 × 10⁷ cells per cm³. The final value is 400 times the first. On ordinary graph paper, plot log₁₀ values: log₁₀(2.0 × 10⁵) = 5.30 and log₁₀(8.0 × 10⁷) = 7.90. Label the axis log₁₀ (cells per cm³). A reading of 7 means 10⁷ cells per cm³. On log paper, plot the original values and label the axis cells per cm³, on a log scale. Read the number directly. Each…
Match the graph and test to the data
The table, graph and statistical test must fit the data you collect. State the graph type in words as well as drawing it, because a sketch can be unclear. Two groups, such as two seed species: bar chart of means, with the y-axis starting at zero; use a Student’s t-test to compare the means. Counts in categories, such as woodlice choosing damp or dry: bar chart of counts; use chi-squared to compare observed counts with expected counts. Two variables, such as temperature and heart rate: line graph or scattergram; use Spearman’s rank to test for correlation.
Turn a test result into a conclusion
For Spearman’s rank, compare the size of the result with the critical value, ignoring the minus sign. If the result is at or above the critical value, reject the null hypothesis. Then name both variables and state the biological relationship. For oil-contaminated soil, rₛ = −0.93, n = 8 and the critical value at p = 0.05 is 0.738. Since 0.93 > 0.738, reject the null hypothesis of no significant correlation. There is a 5% probability or less that a correlation this strong is due to chance. Conclusion: there is a significant negative correlation between time and the percentage of oil remaining.…