RockLicker · Grade 7 · Lesson 5 of 5

When a Sample Misleads

Compare two sampling methods and explain how selection bias limits a population claim.

  • Grade 7
  • 55 minutes
  • Worksheet + teacher guidance
Back to Grade 7

What you will learn

A sample selected for a striking feature may not represent the whole area. Recording the selection method and denominator helps others judge a percentage.

Words to use

  • sample: a subset observed
  • bias: systematic selection distortion
  • denominator: total used for a proportion

Evidence for this lesson

Illustrative data: Team A chose 12 appealing pebbles, preferring shine; 9 were shiny. Team B observed every fifth visible pebble along one short path section; 3 of 12 were shiny. No other sections were surveyed.

Use the supplied record as written; do not invent missing results.

Work through the activity

  1. 10 min · Compare methodsRead how teams A and B chose twelve pebbles each. Identify the preference built into Team A’s method.
  2. 15 min · CalculateFind the percentage of shiny pebbles for each team. Use all twelve as the denominator, not only the shiny subset.
  3. 20 min · Evaluate claimsDiscuss why Team A cannot support “75% of the whole path is shiny.” Team B’s method is more systematic but one short section is still limited.
  4. 10 min · RedesignPropose several separated observation sections, a consistent rule and a way to record non-shiny samples. Use photos or in-place observations where collection is restricted.
RockLicker
GRADE 7 · LESSON 5

When a Sample Misleads

Draw on the printed sheet or write, type or dictate your response.
Draw on the printed sheet or write, type or dictate your response.
Draw on the printed sheet or write, type or dictate your response.
Draw on the printed sheet or write, type or dictate your response.

Responses stay in this open page only. Print before closing or changing pages; no student record is stored online.

Check your understanding

Team A selected shiny pebbles. Its percentage best describes …

Teacher & family guide · answers and assessment

Grade 7 · When a Sample Misleads

Learning goal: Compare two sampling methods and explain how selection bias limits a population claim.

Expected responses

  1. The team deliberately preferred shiny-looking pebbles.
  2. A: 9/12 = 75%; B: 3/12 = 25%.
  3. B follows an every-fifth-visible rule in one section; visibility, location and small sample size still limit representation.
  4. Observe multiple sections with a consistent rule and record every eligible observation, including non-shiny ones.

Understanding check: that selected sample. A preference-selected sample cannot justify an unbiased regional percentage.

Watch for this misconception

A systematic rule reduces some bias; it does not guarantee a representative sample of an entire region.

Assess the evidence

Look for the observations, calculation or explanation named in the learning goal. Accept accurate drawings and dictated responses. Use the expected responses to discuss reasoning; a correct guess without explanation is a starting point for another conversation.

Adapt and extend

Read prompts aloud, enlarge print or let a partner record the learner’s explanation. Compare results across three imaginary sections and look for local differences.