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Design of Statistical Investigations

Design of Statistical Investigations. 1 General Introduction. Stephen Senn. Course Outline. General Introduction Experiments Observational studies Sample surveys (and other sampling schemes). NB Each of these fields is huge and all that is attempted is a brief introduction.

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Design of Statistical Investigations

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  1. Design of Statistical Investigations 1 General Introduction Stephen Senn SJS SDI_1

  2. Course Outline • General Introduction • Experiments • Observational studies • Sample surveys (and other sampling schemes) NB Each of these fields is huge and all that is attempted is a brief introduction SJS SDI_1

  3. General Warning • Your lecturer is not equally experienced in these fields • I know more about experimental design than the other two • Examples from my personal experience tend to be drawn from pharmaceutical research and development or other medical applications SJS SDI_1

  4. Example Exp_1A Simple Experiment • Four experimental p38 kinase inhibitors • Vehicle and marketed product as controls • Thrombaxane B2 (TXB2) is used as a marker of COX-1 activity (low values bad) • Six rats per group were treated for a total of 36 rats • At the end of the study rats are sacrificed and TXB2 is measured. SJS SDI_1

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  6. Specific Features of this Design • Several experimental treatments • Two controls • active • neutral • Six replicates per treatment • Several tests compounds • no ordering • No blocks • rats considered exchangeable The meaning and relevance of these terms will be explained during the course SJS SDI_1

  7. Example Obs_1An Observational Study • Case-control study (Fine et al Lancet, 1986, Quoted in Clayton and Hills) • Does BCG protect against leprosy? • BCG scar status in a population survey were available • Data from 260 leprosy cases were obtained SJS SDI_1

  8. Fine et al SJS SDI_1

  9. Case-Control Study • Note that this is sampled by outcome • The number of these is fixed • Exposure is measured • In a clinical trial, patients are assigned the exposure (the treatment) • The outcome is measured • An experiment involves manipulation • Case-control does not SJS SDI_1

  10. Example Surv_1A Sample Survey • Population of pharmaceutical record forms in Pembury Hospital, Tunbridge Wells • Thousands of such forms available • A sample of 108 forms was chosen from patients discharged between 1 July and 31 December 1976 • Records chosen at fixed intervals • Number of prescriptions recorded on each was noted SJS SDI_1

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  12. Purposes of Statistical Investigations SJS SDI_1

  13. Problem to Bear in Mind • We can only study past/present • We can construct formal theories of inference only about the past/present • We often wish to make inference about the future • This requires an ‘extra-statistical’ element • Most naively an assumption that the future is like the past SJS SDI_1

  14. Example • The effect of streptomycin on TB • Trial carried out by Austin Bradford Hill and colleagues 1947 • Treatment highly effective • Is it still as effective? SJS SDI_1

  15. Experiments Causal purpose Convenient material Allocation of treatments crucial Randomisation Sampling Descriptive purpose Representative material Choice of sample members crucial Random sampling Experimentation v Sampling SJS SDI_1

  16. Caution • These two are sometimes confused • The growth of modelling approaches tends to increase the confusion • Experiments rarely use representative material • Surveys (and other samples) usually do. SJS SDI_1

  17. Basic Design Cycle Objective Possible Conclusions Tentative Design Potential Data Possible Analysis Relevant factors SJS SDI_1

  18. Questions 1Exp_1Rat TXB2 • How do you decide which rat gets which treatment? • How would you analyse these data? • What use will be made of these data? SJS SDI_1

  19. Questions 2Obs_1Fine et al • What difference would it make to the precision of the conclusions if the population survey had been smaller? • What difference would it make if there had been fewer leprosy cases? • How would you test for an association between BCG and leprosy? • What interpretations are there for an association? SJS SDI_1

  20. Questions 3Surv_1Pharmaceutical Record Forms • What is a simple random sample? • In this specific case how would one choose such a sample?* • Suppose that the sample of 108 forms was chosen from 5,000. What should the size of the sample have to be if there were 10,000 to choose from? * The sample chosen in this example was not a simple random sample SJS SDI_1

  21. Suggested Reading Experimental Design: Mead, R. The Design of Experiments, Cambridge University Press, Cambridge, 1988 Clarke, G.M and Kempson, R.E. Introduction to the Design and Analysis of Experiments, Arnold, London, 1997. Case-control Studies, Breslow and Day, 1980, Statistical Method in Cancer Research, vol 1 Sampling Hague and Harris, Sampling and Statistics, Kogan Page (This is a very elementary book.) S-PLUS Krause, A and Olson, M The Basics of S and S-PLUS (2nd edition) , Springer, 2000 SJS SDI_1

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