Biostatistics for Med Students. Lecture 2

Transcription

1 Biostatistics for Med Studets Lecture 2 Joh J. Che, Ph.D. Professor & Director of Biostatistics Core UH JABSOM JABSOM MD7 February 22, 2017 Lecture Objectives To uderstad basic research desig priciples ad data presetatio approaches To build a foudatio which will facilitate the active participatio i cliical research To fully grasp descriptive statistics To itroduce key cocepts of iferetial statistics To survey some commoly used statistical approaches To be prepared for the USMLE Step 1 biostat/epi questios 1

2 Outlie Lecture 1 (02/15/2017) The goal of statistics Itroductio to descriptive biostatistics Basic research desig priciples ad data presetatio approaches Lecture 2 (02/22/2017) Itroductio to iferetial statistics Commoly used statistical approaches The Goal of Statistics Samplig Descriptive Statistics Populatio [Parameters] (, ) Probability Iferece (Iferetial Statistics) Sample [Statistics] ( X, S) Descriptive Statistics 2

3 Effects ad Variability Birth Weight Birth Weight Y 3 Y 3 Y Y 1 Y Y 1 Y 2 Y 2 Group A Group B Group C Group A Group B Group C Note: Biological/cliical sigificace vs. statistical sigificace The Normal Distributio 50% 50% X ~ N (µ, 2 ) µ - 2 µ - µ µ + µ + 2 X 3

4 The Normal Distributio Stadard ormal distributio: Z ~ N( µ = 0, 2 = 1) 50% 50% Z Give X ~ N(µ, 2 ), we have Z=(X - µ)/. Stadard Normal Table (From Zero to Z Uder The Normal Curve) z µ=0 Z

5 AUC For Normal Distributio The Rule of Thumb: Withi oe s.d.: 68.27% (2/3) Withi two s.d.: 95.45% (95%) Withi three s.d.: 99.74% (99%) Samplig Distributio The distributio of idividual observatios versus the distributio of sample meas:.... Sample size: X populatio X 1 X 2 X 3 X X 5

6 Cetral Limit Theorem The distributio of sample meas (samplig distributio) from a populatio is approximately ormal as log as the sample size is large, i.e., 2 X X ~ N ( X, X ) Z ~ N(0,1) / 1. The populatio distributio ca be o-ormal. 2. Give the populatio has mea, the the mea of the samplig distributio,. X 3. If the populatio has variace 2, the stadard deviatio of the samplig distributio, or the stadard error (a measure of the amout of samplig error) is s. e.( X ). X Hypothesis Testig A Example: Normal serum creatiie level depeds o the populatio studied. From the literature a 4 th year JABSOM med studet foud that oe well-established study showed a average scr of 0.56 (with a stadard deviatio of 0.15 mg/dl) for 2 d trimester Caucasia pregat wome livig o the east coast. But based o her kowledge ad experiece, she believed that the µ of scr amog Japaese pregat wome i Hawaii seemed differet. She decided to test this by measurig scr of 49 local Japaese 2 d trimester pregat wome. 6

7 Hypothesis Testig (cot.) Rejectio regio: (1- ) No-rejectio regio µ 0 Critical value X Hypothesis Testig Basic steps of hypothesis testig: 1. State ull (H 0 :) ad alterative (H 1 :) hypotheses 2. Choose a sigificace level, α (usually 0.05 or 0.01) 3. Determie the critical (or rejectio) regio ad the o-rejectio regio, based o the samplig distributio ad uder the ull hypothesis 4. Based o the sample, calculate the test statistic ad compare it with the critical values 5. Make a decisio, ad state the coclusio 7

8 Oe-tailed vs. Two-tailed Test Right Sided Test Left Sided Test Two Sided Test Statistical Decisio: Errors & Power Truth H0 True H0 False Decisio Reject H0 1- Not reject H0 1- Type I Error ( ) - Type II Error ( ) - False positives, errors due to chace; Reject H 0 whe H 0 is true False egatives; Do t reject H0 whe H1 is true Power: ( 1- ) = 1 - P (Type II Error) 8

9 Desig factors: - effect differece - power - alpha level - std. dev. - sample size Statistical Decisio µ 0 µ 1 X Now, what is a p-value? Iterpretatio: p-values The p-value is the probability of obtaiig a result as extreme or more extreme tha the oe observed based o the curret sample, give the ull hypothesis is true. Note: Statistically sigificat does ot ecessarily mea biologically (or cliically) sigificat!!! 9

10 Hypothesis Testig (cot.) Example (cot.): Say, the average scr of the sample of 49 locals is 0.60 mg/dl ad the populatio stadard deviatio is 0.15 mg/dl (based o the literature). Step 1. State H 0 : ad H 1 : H 0 : μ scr = 0.56 vs. H 1 : μ scr 0.56 Step 2. Choose a sigificat level, say, α =0.05. Step 3. Calculate the test statistic: Z X / scr / 49 Hypothesis Testig (cot.) Step 4. Determie the critical regio ad the o-rejectio regio: The critical value: ± The rejectio regio: Z The o-rejectio regio: Z < Step 5. Make a decisio, based o the sample, ad state the coclusio: As the test statistic Z = 1.87 < 1.96, it is withi the o-rejectio regio. Therefore, we do ot reject the ull hypothesis. We coclude that there is o evidece that the average scr amog local Japaese 2 d trimester wome is differet from 0.56 mg/dl. 10

11 Example (cot.): Cofidece Itervals A med studet wated to determie the average serum creatiie level amog secod trimester pregat Japaese wome i Hoolulu. From the literature she foud that for similar populatios is about 0.15 mg/dl, but she could ot fid ay iformatio o µ of scr amog local pregat wome. She measured 49 Japaese 2 d trimester wome ad the sample mea scr was 0.60 mg/dl. What should be the 95% CI for µ? Cofidece Itervals CIs for µ: 90% CI : X % CI : X % CI : X

12 The t - Distributio A small sample from ormal distributio Ukow populatio stadard deviatio, X -µ t= with -1 degrees of freedom (d.f.). s/ The (Studet s) t-distributio is very similar to ormal distributio, with heavier tails. t Table (tail probabilities of the t-distributios) Degrees of Freedom 2Q (Q) 0.10 (0.05) 0.05 (0.025) 0.01 (0.005) (0.0025) (0.0005)

13 Oe Sample t Test Problem: Neoates gai (o average) 100 grams/wk i the first 4 weeks. A sample of 25 ifats give a ew utritio formula gaied 112 grams/wk (o average) with stadard deviatio = 30 grams. Is this statistically sigificat? Test: H0: µ = 100 H1: µ = 100 Solutio: Oe Sample t - Test No-rejectio Regio t= / 25 = 2.0 < t 24, = 2.06 Therefore, do ot reject (p-value=0.057). 13

14 Two Idepedet Sample t - Test Populatio 1 (µ 1, ) Sample 1 (X 1, s 1 ) Populatio 2 (µ 2, ) Sample 2 (X 2, s 2 ) Test: H0: µ 1 = µ 2 versus H1: µ 1 µ 2, assumig 2 1 = 22 = 2. Two Idepedet Sample t - Test Test statistic: t= (x1 x2) (µ 1 -µ 2 ) with ( )d.f. Sx 1 x s 1 ( 1 1) + s 2 ( 2 1) Sp = Sx 1 x 2 = Sp ( s X s s 1 ) 14

15 Two Idepedet Sample t - Test Problem: Two headache remedies Brad A: X1 = 20.1, s 1 = 8.7, 1 = 12 Brad B: X2 = 18.9, s 2 = 7.5, 2 = 12 Test: H0: µ1 = µ2 µ1 - µ2 = 0 H1: µ1 = µ2 Two Idepedet Sample t - Test Solutio: t= x1 x2 = Sx1 x Sx1 x2 = 1.2 Sx1 x2 2 (12 1) ( 12 1) 7.5 Sp = = = Sp = = 8.12 Sx1 x2 = 8.12* = 3.3 t = =

16 Two Idepedet Sample t - Test Solutio (cot.): Therefore, do ot reject the ull, i.e., o statistically sigificat differece was foud betwee the two remedies t = = 0.36, 3.3 t 0.05, 22 = 2.07 Two Paired Sample t - Test Populatio 1: µ 1 Sample 1 Populatio 2: µ 2 Sample 2 Test: H0: µ 2 -µ 1 = 0 versus H1: µ 2 - µ

17 Two Paired Sample t - Test Same subject for both treatmets: -- placebo (X 1 ) versus active (X 2 ) -- before (X 1 ) versus after (X 2 ) Itra idividual compariso, e.g., left (X 1 ) versus right (X 2 ) Approach: reduce data to oe sample t-test problem. First, calculate the differece, d = X 2 -X 1, for each subject; the, perform oe sample t-test o the d scores, with d.f.=-1. Two Paired Sample t - Test Test: H 0 : average differece is zero Test statistic: t = d - 0 Sd d = i di Sd = Sd / Sd = ( d d)

18 Two Paired Sample t - Test Problem: Does the medicatio sigificatly lower blood pressure? Subject Reactio to Placebo Reactio to Med Two Paired Sample t - Test Subject Reactio to Placebo Reactio to Medicatio Total 120 d 18

19 Two Paired Sample t - Test Sd = i di Solutio: d = ( d d) 2-1 = 120 = 24 5 = 22 Sd = Sd / = 22 / 5 = 2.1 t = d - 0 Sd = = 11.4 Two Paired Sample t - Test At 5% level, t - table = For a two-sided test: reject! There is a statistically sigificat effect! 19

20 Cotigecy Tables The most commo two-way tables: 2-by-2 tables For example, smokig ad lug cacer status Explaatory variable Smokig Yes No Lug cacer Yes No Measures of associatio RD: risk differece (prospective studies) RR: relative risk (prospective studies) OR: odds ratio (prospective or retrospective) Respose variable RR RD OR Iterpretatios Iterpretatio of estimates of RR/OR: If RR/OR is above 1, we say there is a positive relatioship/associatio betwee risk factor ad outcome. If RR/OR is below 1, we say there is a egative relatioship/associatio betwee risk factor ad outcome. Iterpretatio of CI of populatio RR/OR: If the etire iterval is above 1, we coclude that the probability that havig the outcome of iterest is higher i those with risk factor. If the etire iterval is below 1, we coclude that the probability is lower i those with risk factor. We coclude that there is o evidece that the probabilities of the outcome of iterest are differet betwee those with vs. without risk factor, if the iterval cotais 1. 20

21 2 Tests Expected ad observed frequecies are compared Goodess of Fit of a sigle variable Test of Idepedece of two variables Percetiles of the 2 - Distributios Table Degrees of freedom

22 2 Observed: Tests of Idepedece C1 C2 R1 A C A+C R2 B D B+D A+B C+D A+B+C+D Expected: E = (row total) * (colum total) (grad total) d.f. = (r-1)*(c-1) 2 Tests of Idepedece Problem: Is the NQO1 gee associated with edemic ephropathy (EN)? Observed: NQO1 Mutat NQO1 WT EN cases EN cotrols

23 2 Tests of Idepedece Solutio: O E 50? 10? 20? 20? * 70 E = 100 = 42 2 Solutio: Tests of Idepedece 2 2 O E (O-E) (O-E) (O-E) / E with (R-1) * (C-1) d.f. 2 = (2-1) * (2-1) d.f. = 1 d.f. 2 =

24 2 Tests of Idepedece Solutio: 2 = , = NQO1 is ot idepedet of EN (a associatio exists), p < Fisher s Exact Tests Fisher s Exact Test for small expected frequecies If ay expected frequecies are < 2 or if half of the expected frequecies are < 5, you should use Fisher s Exact Test istead of 2. 24

25 Fisher s Exact Tests Fisher s Tea Tastig Experimet Poured First (Guessed) Milk Tea Poured First (Actual) Milk Tea Fisher s Exact Tests Fisher s Tea Tastig Experimet (cot.): Based o hypergeometric distributio, the p-value is the sum of all probabilities for tables that give eve more evidece i favor of the lady s claim. Poured First (Actual) Poured First (Guess) Milk Tea Milk 0 4 Tea 4 0 Poured First (Guess) Poured First (Guess) Milk Tea Milk Tea Milk 3 1 Milk 4 0 Tea 1 3 Tea 0 4 p-value = P (1,1) (3) + P (1,1) (4) = = Therefore, the experimet did ot establish a sigificat associatio betwee the actual order of pourig ad the woma s guess. 25

26 A Roadmap of Commo Statistical Tools Descriptive Statistics Populatio Parameters (, ) Samplig Probability Iferece (Iferetial Statistics) Outcome Descriptive Statistics Sample Statistics (X, S) Normal, t, ad Chi-squared Samplig distributio Cetral Limit Theorem CI, hypothesis testig p-value, Type I & II error Cotiuous variable Cotiuous idepedet variables Liear regressio Time to evet data Survival aalysis Oe sample t-test Two samples t-test Geeral liear models Categorical idepedet variables Aalysis of variace (ANOVA) If a mixture of cot. ad categ. idep. variables, If o distributio assumptios, No-parametrics If paired Categorical variable Oe categorical variable Chi-sq GOF test Two categorical variables Chi-sq idepedece test Fisher s exact test( exp.) Mixture of idepedet variables Logistic regressio (biary outcome) Others: logitudial aalysis, factor aalysis, cluster aalysis, pricipal compoet aalysis, etc. Collaboratio with A Biostatisticia 1. Early ad ofte 2. Start the discussio whe you have the iitial idea 3. It is a iterative process 4. A collaborative effort: equal ad fair 5. Ask questios so you ca discuss about the geeral statistical approach without the statisticia 6. Educatio ad traiig i research desig ad biostatistics NIH U54MD NIH G12MD NIH P20GM

27 Sample USMLE Step 1 Questios: Questio 4. The stadard error of the mea: a. is less tha the stadard deviatio of the populatio. b. decreases as the sample size icreases. c. measures the variability of the mea from sample to sample. d. all of the above (a, b, ad c) e. oe of the above (a, b, or c) Sample USMLE Step 1 Questios: Questio 5. I a hypothesis test, the probability of obtaiig a value of the test statistic equal to or more extreme tha the value observed, give that the ull hypothesis is true, is referred to as: a. Type I error. b. The p-value. c. Statistical power. d. Type II error. e. Critical value. 27

28 Sample USMLE Step 1 Questios: Questio 6. The power of a statistical test is the probability of rejectig the ull hypothesis whe it is. Whe you icrease alpha, the power of the test will. a. true / decrease b. false / icrease c. true / icrease d. false / decrease e. true / stay costat 28