WBI16 Practical Skills in Biology II

Lessons in WBI16

1. The Unit 6 paper: what is new at A2
   50 marks, 80 minutes, CP10 to CP18 plus synoptic contexts. New at A2: null hypotheses, statistical tests (t-test, chi-squared, Spearman's rank), calibration, et
2. Hypotheses, null hypotheses and significance
   H₀: no difference, correlation or association, named in context. At p = 0.05: calculated value ≥ critical value, reject H₀ (significant); less than critical val
3. Choosing the right statistical test
   Difference between two means: t-test. Correlation between paired variables: Spearman's rank (5+ pairs). Observed vs expected counts in categories: chi-squared.
4. Describing data: mean, median, mode and standard deviation
   Mean (all data), median (middle, resists outliers), mode (most common). s = √[Σ(x - x̄)² / (n - 1)], same units as the data; s² / n goes into the t-test; ±1s co
5. Student's t-test
   t = (x̄₁ - x̄₂) / √(s₁²/n₁ + s₂²/n₂); df = n₁ + n₂ - 2; compare |t| with the p = 0.05 critical value. t ≥ critical: reject H₀ (significant difference).
6. Spearman's rank correlation
   rₛ = 1 - 6Σd² / n(n² - 1). Rank each variable, share ranks for ties, sum d². Compare |rₛ| with the critical value for n pairs at p = 0.05; at least 5 pairs need
7. The chi-squared test
   χ² = Σ(O - E)²/E using counts; df = categories - 1; p = 0.05 critical values 3.84 (1 df), 5.99 (2 df), 9.49 (4 df). χ² ≥ critical: reject H₀.
8. Planning a full investigation at A2
   Plan: preliminary work for range, time and concentrations; specific method with DV, IV, standardised organisms, controlled variables with methods, acclimatisati
9. Calibration, standards and serial dilutions
   Calibrate: colorimeter with a blank and standards, pH probe with buffers, graticule with a stage micrometer. Calibration curve = readings of known standards use
10. Ethics, safety and working with living organisms
    Ethics: prefer invertebrates, minimise number and harm, return organisms; humans need informed consent, right to withdraw and health screening. Safety: give a s
11. Logarithms and logarithmic graphs
    log₁₀ handles values spanning orders of magnitude; exponential growth is a straight line on a log scale. Number = 10^log. Generations n = (log N - log N₀) / log
12. Units, rates and constants from data
    Rates need quantity per time units; correct mixed or missing units. V = πr²l. Q10 = rate at T + 10 ÷ rate at T (convert times to rates first). Constants come fr
13. Presenting and analysing data at A2
    Raw data tables with units in headings; calculated values separate. Two conditions: bar chart + t-test; many values: scatter + Spearman's rank or gradient; coun
14. Evaluating investigations: errors, uncertainty and improvements
    % uncertainty = uncertainty ÷ reading x 100; add % uncertainties when multiplying or dividing. Evaluate: name the limitation, its effect and its link to the con
15. Skills applied: CP10 photosynthesis, CP12 temperature and development
    CP10: 1/d², V = πr²l, limiting factors, heat shield and acclimatisation, gas not pure oxygen. CP12: rate = 1/time, Q10, same batch of organisms, thermostatic co
16. Skills applied: CP11 ecology of a habitat
    Random sampling for uniform areas, transects for gradients; enough quadrats when the running mean levels off. Percentage cover, density or frequency; abiotic re
17. Skills applied: CP13 microbial growth, CP14 antibiotics
    CP13: turbidity, haemocytometer or dilution plating; cells per cm³ = colonies ÷ volume x dilution factor; log graphs. CP14: lawn plates, control disc, zone area
18. Skills applied: CP15 yeast respiration, CP16 respirometers and RQ
    CP15: rate = 1 / time for methylene blue to decolourise; equilibrate, do not shake, use a colour standard. CP16: KOH absorbs CO₂; V = πr²l; control tube with be
19. Skills applied: CP17 spirometer traces, CP18 amylase in cereal grains
    CP17: tidal volume (peak to trough), breathing rate, ventilation = TV x rate, O₂ use = fall in trace ÷ time; consent and screening. CP18: embryo-free half-grain
20. Recommended additional practical: habituation to a stimulus
    Habituation: a reduced response to a repeated harmless stimulus (fewer Ca²⁺ channels open, less neurotransmitter released). Standardise the stimulus (place, for

Next exams for WBI16

• January 2027: Fri 22 Jan 2027, morning, 1h 20m
• Summer 2027: Wed 9 Jun 2027, morning, 1h 20m

From Pearson's final timetables. All exam dates. Always confirm your entries with your exams officer.