STAT 1301 Chapters 1-2 ExperimentationandObservation

Inductive Inference - a cornerstone of scientific investigation Using specific observations to reach general conclusions Using specific observations to reach general conclusionsExamples ñ tasting a small spoonful of soup ñ responses from a sample of registered voters used to predict the outcome of an election

SCIENTIFIC METHOD Notice a phenomenon Formulate an Explanation Based on this Model, Outline What Should Occur Design an Experiment Take More Data If consistent If contradictory OBSERVATIONHYPOTHESISPREDICTIONVERIFICATION

Salk Vaccine Field Trial   first polio epidemic in U.S. Polio widespread for next 40 years Paradox: countries with poor hygiene had almost no polio   Vaccines were developed - needed to be tested   study of 2 million children: Suppose they gave the vaccine to everybody and the incidence of polio went down. What would this prove?

, ,000 and this is before the vaccine

How should the Public Health Service test the new vaccine?   POSSIBLE DESIGN Treatment Group – –those children whose parents consent Control Group – –No parental consent W W Confounding Treatment and control groups differ with respect to some factor other than treatment

General Principle For fair comparison, the control and treatment groups should be chosen from same population - parental consent

Other Design Considerations u uHow should children be assigned to treatment and control groups? - judgment? - randomly? u uShould children know which group they are in? - placebo (pseudo-treatment) u uShould doctors know which group the children are in? - double-blind

Results Double-Blind Randomized Controlled Study SizeRate Treatment 200, Control 200, No consent350, Rate = Number of cases per 10,000

Scientific Method - Polio Example Lower incidence of polio in under- developed countries Children get immunity from mother and contract polio at an early age while immunity is in effect Vaccine will provide same sort of immunity Salk Vaccine Studies OBSERVATIONHYPOTHESISPREDICTIONVERIFICATION

Method of Comparison Assignment into groups: Controlled Experiment -- investigator makes assignment -- randomized assignment is best Observational Study -- “natural” assignment into groups

Gender Bias - Graduate Division, UC Berkeley l 8442 men applied, 44% were admitted 4321 women applied, 35% were admitted Is it correct to say the university prefers men to women? 4321 women applied, 35% were admitted Is it correct to say the university prefers men to women? MEN WOMEN MEN WOMEN Number of Percent Number of Percent Major applicants admitted applicants admitted Major applicants admitted applicants admitted A B C D E F

Hypothetical Example AdmitDenyPercent Mathematics Males901090% Females91 90% English Males1910% Females % Combined Males911983% Females199117%

Lesson : Careful design of experiments is very important! controlled experiments preferred over observational studies -- anecdotal evidence -- anecdotal evidence - - extreme case of observational studies - not very convincing

An Observational Study Ultrasound and Low Birthweight u uHuman babies can be examined in the womb prior to birth via ultrasound. u uInvestigators found lower birthweights for babies who had been exposed to ultrasound. Is this evidence that ultrasonic exam causes lower birthweight? u uNo. Doctors prescribed the exam when something seemed wrong. Ultrasound effect is confounded with problem pregnancies. u uLater, a controlled, randomized experiment found no decrease in birthweight due to ultrasound.

A Swine Flu Vaccination Program u uFirst shots given to group most at risk - elderly and infirm. u uIn the first week - 24,000 persons aged 65 and up were given the shot and 3 died. (i.e. deathrate was 3/24,000= ). 8 states suspended the program because of deaths. u uPROBLEM ? - No Control (The national average weekly death rate for individuals aged is about 8 per 10,000 or )