1. University of Durham D Dr Robert Coe University of Durham School of Education Tel: (+44 / 0) 191 334 4184 Fax: (+44 / 0) 191 334 4180 E-mail: r.j.coe@dur.ac.uk http://www.dur.ac.uk/r.j.coe Sampling and Representativeness Doctor of Education (EdD) Analysing, Interpreting and Using Educational Research (Research Methodology)

2. © 2005 Robert Coe, University of Durham 2 Generalisability Representative of what? Identifying and defining a ‘population’ Sampling frame Different notions of representativeness Statistical (quantitative) Exemplification (qualitative)

3. © 2005 Robert Coe, University of Durham 3 Sampling strategies (types of sample) Probability samples (statistically representative) Random Stratified random Cluster / multi-stage random Non-probability samples (representative in other ways) Maximum variability Purposive Quota Opportunity samples (not really representative) Convenience (accidental, opportunity) Snowball

4. © 2005 Robert Coe, University of Durham 4 Response rates Are affected by Saliency (relevance, importance, interest) of the topic Number of reminders / contacts Appeals to self-interest / utility Incentives May be affected by Length (mixed findings) Not affected by Day of posting Type of stamp Colour of ink / paper Offer of anonymity Offer of feedback McColl et al., 2001

5. © 2005 Robert Coe, University of Durham 5 Effects of different strategies to increase response rates Changes response rate from 50% to: Edwards et al., 2002

6. © 2005 Robert Coe, University of Durham 6 Non-response bias Suppose you get a 70% response rate Half your respondents (50%) ‘agree’ But those who ‘agree’ were 20% more likely to respond than those who ‘disagree’ (81% vs 61% response) Then is actually

7. © 2005 Robert Coe, University of Durham 7 Sample size and power

8. © 2005 Robert Coe, University of Durham 8 Sample size needed to estimate a proportion with a given accuracy Sample proportion = 50% With a sample of 100 in which 50% ‘agree’, the actual proportion in the population is likely to be between 40% and 60% Sample proportion = 85% In a primary school with 30 Y6 pupils, 85% achieve level 4. The margin of error should be between 68% and 94%