A-Level Edexcel Biology Scatter Diagrams and Correlation

Review A-Level Edexcel Biology scatter diagrams and correlation through graph sketches, table comments and cautious interpretation of biological relationships.

Exam points

sketch correlation graphs with correct axes and slope for biological relationships
comment on positive, weak or absent correlations without claiming causal links
suggest biological or environmental reasons when table variables show no correlation

[Maximum number: 2]

Mycobacterium tuberculosis (Mtb) and human immunodeficiency virus (HIV) are two pathogens that infect people. When people infected with either pathogen become infected with the other pathogen the disease is worse.

The graph shows the number of people who have TB and the percentage of the population who have HIV. Each plotted point represents data from a different country. Number of people who have TB per 100000 population

Figure for Question A.1.7—Scatter diagrams and correlation question 1 — Edexcel A-Level Biology A2

Comment on the relationship between the number of people who have TB and the percentage of people who have HIV.

An answer that includes two of the following points:

- (overall) positive correlation / as percentage of people with HIV increases so does number of people with TB (1)

- \{which is less clear cut / greater spread / weaker correlation\} between 0 and $\{5$ / 5.5 / 6 / 6.5 \% of HIV (1)

- people who have TB do not necessarily have HIV (1)

ACCEPT directly proportional DO NOT ACCEPT causal relationship

ACCEPT converse comment for above \{5 / 5.5 / 6 / 6.5\} \%

IGNORE some countries have no HIV but do have TB

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A.1.7—Scatter diagrams and correlation question 1