Producing Data, Randomization, and Experimental Design Goals

Total Page：16

File Type：pdf, Size：1020Kb

Producing Data, Randomization, and Experimental Design Goals Identify observational studies versus experiments Design experiments to test hypotheses using appropriate randomization Use the random number tables to assign subjects correctly to experimental groups Define, use, and know the concepts behind all the new vocabulary words Starting with a research question Often can’t simply study the whole population If you want to know the life expectancy for cancer patients you simply can’t identify all patients and then wait for them to die Use a sample to draw conclusions about the whole Observational Study vs. Experiment Observational study observes and measures variables of interest. Experiment imposes a treatment in order to observe outcome New Terminology Population Sample Voluntary response sample Convenience sampling Bias Simple random sample (SRS) Use Random Numbers to Generate SRS Label all the individuals in a population with numerical labels. Use random number table (or statistical package) to choose individuals randomly. Example: To divide 100 students into two groups of 50 label them 00 to 99 and go through the table starting at a random line until the first 50 have been chosen for a group. Other Sampling Designs Probability sample Individuals chosen with some given probability Stratified random sample Population divided into strata and individuals chosen at random from each strata Multistage random sample Sample chosen in a number of stages Problems Undercoverage: some groups have no chance of being sampled; as in phone polling Nonresponse: individuals chosen cannot or will not participate Response bias: people may lie about illegal or embarrassing behavior; may respond to the questioner Wording of questions may effect the outcome Designing Experiments Experimental units or subjects Treatment: experimental condition imposed Factors: explanatory variables Level: value of a given factor Example: Television ads Comparative Experiments Compare 2 or more groups Use a control group to eliminate confounding and placebo effect A randomized comparative experiment uses comparisons between two (or more) groups and randomization of subjects into treatment groups. Design of a randomized comparative experiment Principles of Experimental Design Control effects of lurking variables via comparison of several treatments Randomization to assign units to treatment Replication of experiment on many units to reduce chance variation .

Copy Link

Recommended publications

Analysis of Variance and Analysis of Variance and Design of Experiments of Experiments-I
Analysis of Variance and Design of Experimentseriments--II MODULE ––IVIV LECTURE - 19 EXPERIMENTAL DESIGNS AND THEIR ANALYSIS Dr. Shalabh Department of Mathematics and Statistics Indian Institute of Technology Kanpur 2 Design of experiment means how to design an experiment in the sense that how the observations or measurements should be obtained to answer a qqyuery inavalid, efficient and economical way. The desigggning of experiment and the analysis of obtained data are inseparable. If the experiment is designed properly keeping in mind the question, then the data generated is valid and proper analysis of data provides the valid statistical inferences. If the experiment is not well designed, the validity of the statistical inferences is questionable and may be invalid. It is important to understand first the basic terminologies used in the experimental design. Experimental unit For conducting an experiment, the experimental material is divided into smaller parts and each part is referred to as experimental unit. The experimental unit is randomly assigned to a treatment. The phrase “randomly assigned” is very important in this definition. Experiment A way of getting an answer to a question which the experimenter wants to know. Treatment Different objects or procedures which are to be compared in an experiment are called treatments. Sampling unit The object that is measured in an experiment is called the sampling unit. This may be different from the experimental unit. 3 Factor A factor is a variable defining a categorization. A factor can be fixed or random in nature. • A factor is termed as fixed factor if all the levels of interest are included in the experiment.
[Show full text]
How to Randomize?
TRANSLATING RESEARCH INTO ACTION How to Randomize? Roland Rathelot J-PAL Course Overview 1. What is Evaluation? 2. Outcomes, Impact, and Indicators 3. Why Randomize and Common Critiques 4. How to Randomize 5. Sampling and Sample Size 6. Threats and Analysis 7. Project from Start to Finish 8. Cost-Effectiveness Analysis and Scaling Up Lecture Overview • Unit and method of randomization • Real-world constraints • Revisiting unit and method • Variations on simple treatment-control Lecture Overview • Unit and method of randomization • Real-world constraints • Revisiting unit and method • Variations on simple treatment-control Unit of Randomization: Options 1. Randomizing at the individual level 2. Randomizing at the group level “Cluster Randomized Trial” • Which level to randomize? Unit of Randomization: Considerations • What unit does the program target for treatment? • What is the unit of analysis? Unit of Randomization: Individual? Unit of Randomization: Individual? Unit of Randomization: Clusters? Unit of Randomization: Class? Unit of Randomization: Class? Unit of Randomization: School? Unit of Randomization: School? How to Choose the Level • Nature of the Treatment – How is the intervention administered? – What is the catchment area of each “unit of intervention” – How wide is the potential impact? • Aggregation level of available data • Power requirements • Generally, best to randomize at the level at which the treatment is administered. Suppose an intervention targets health outcomes of children through info on hand-washing. What is the appropriate level of randomization? A. Child level 30% B. Household level 23% C. Classroom level D. School level 17% E. Village level 13% 10% F. Don’t know 7% A. B. C. D.
[Show full text]
Randomized Experimentsexperiments Randomized Trials
Impact Evaluation RandomizedRandomized ExperimentsExperiments Randomized Trials How do researchers learn about counterfactual states of the world in practice? In many fields, and especially in medical research, evidence about counterfactuals is generated by randomized trials. In principle, randomized trials ensure that outcomes in the control group really do capture the counterfactual for a treatment group. 2 Randomization To answer causal questions, statisticians recommend a formal two-stage statistical model. In the first stage, a random sample of participants is selected from a defined population. In the second stage, this sample of participants is randomly assigned to treatment and comparison (control) conditions. 3 Population Randomization Sample Randomization Treatment Group Control Group 4 External & Internal Validity The purpose of the first-stage is to ensure that the results in the sample will represent the results in the population within a defined level of sampling error (external validity). The purpose of the second-stage is to ensure that the observed effect on the dependent variable is due to some aspect of the treatment rather than other confounding factors (internal validity). 5 Population Non-target group Target group Randomization Treatment group Comparison group 6 Two-Stage Randomized Trials In large samples, two-stage randomized trials ensure that: [Y1 | D =1]= [Y1 | D = 0] and [Y0 | D =1]= [Y0 | D = 0] • Thus, the estimator ˆ ˆ ˆ δ = [Y1 | D =1]-[Y0 | D = 0] • Consistently estimates ATE 7 One-Stage Randomized Trials Instead, if randomization takes place on a selected subpopulation –e.g., list of volunteers-, it only ensures: [Y0 | D =1] = [Y0 | D = 0] • And hence, the estimator ˆ ˆ ˆ δ = [Y1 | D =1]-[Y0 | D = 0] • Only estimates TOT Consistently 8 Randomized Trials Furthermore, even in idealized randomized designs, 1.
[Show full text]
How to Do Random Allocation (Randomization) Jeehyoung Kim, MD, Wonshik Shin, MD
Special Report Clinics in Orthopedic Surgery 2014;6:103-109 • http://dx.doi.org/10.4055/cios.2014.6.1.103 How to Do Random Allocation (Randomization) Jeehyoung Kim, MD, Wonshik Shin, MD Department of Orthopedic Surgery, Seoul Sacred Heart General Hospital, Seoul, Korea Purpose: To explain the concept and procedure of random allocation as used in a randomized controlled study. Methods: We explain the general concept of random allocation and demonstrate how to perform the procedure easily and how to report it in a paper. Keywords: Random allocation, Simple randomization, Block randomization, Stratified randomization Randomized controlled trials (RCT) are known as the best On the other hand, many researchers are still un- method to prove causality in spite of various limitations. familiar with how to do randomization, and it has been Random allocation is a technique that chooses individuals shown that there are problems in many studies with the for treatment groups and control groups entirely by chance accurate performance of the randomization and that some with no regard to the will of researchers or patients’ con- studies are reporting incorrect results. So, we will intro- dition and preference. This allows researchers to control duce the recommended way of using statistical methods all known and unknown factors that may affect results in for a randomized controlled study and show how to report treatment groups and control groups. the results properly. Allocation concealment is a technique used to pre- vent selection bias by concealing the allocation sequence CATEGORIES OF RANDOMIZATION from those assigning participants to intervention groups, until the moment of assignment.
[Show full text]
Bias in Rcts: Confounders, Selection Bias and Allocation Concealment
Vol. 10, No. 3, 2005 Middle East Fertility Society Journal © Copyright Middle East Fertility Society EVIDENCE-BASED MEDICINE CORNER Bias in RCTs: confounders, selection bias and allocation concealment Abdelhamid Attia, M.D. Professor of Obstetrics & Gynecology and secretary general of the center of evidence-Based Medicine, Cairo University, Egypt The double blind randomized controlled trial that may distort a RCT results and how to avoid (RCT) is considered the gold-standard in clinical them is mandatory for researchers who seek research. Evidence for the effectiveness of conducting proper research of high relevance and therapeutic interventions should rely on well validity. conducted RCTs. The importance of RCTs for clinical practice can be illustrated by its impact on What is bias? the shift of practice in hormone replacement therapy (HRT). For decades HRT was considered Bias is the lack of neutrality or prejudice. It can the standard care for all postmenopausal, be simply defined as "the deviation from the truth". symptomatic and asymptomatic women. Evidence In scientific terms it is "any factor or process that for the effectiveness of HRT relied always on tends to deviate the results or conclusions of a trial observational studies mostly cohort studies. But a systematically away from the truth2". Such single RCT that was published in 2002 (The women's deviation leads, usually, to over-estimating the health initiative trial (1) has changed clinical practice effects of interventions making them look better all over the world from the liberal use of HRT to the than they actually are. conservative use in selected symptomatic cases and Bias can occur and affect any part of a study for the shortest period of time.
[Show full text]
Randomization Lists
PASS Sample Size Software NCSS.com Chapter 880 Randomization Lists Introduction This module is used to create a randomization list for assigning subjects to one of up to 25 treatment groups. The list can be stratified by up to 25 centers (strata) with an additional 2 stratification factors, each with up to 25 levels. Seven randomization algorithms are available. Five of the algorithms (Block Randomization, Efron’s biased coin randomization, Smith’s randomization, Wei’s urn randomization, and random sorting using maximum allowable % deviation) are designed to generate balanced random samples throughout the course of an experiment. The other two (complete randomization and random sorting) are less complex but have higher probabilities of imbalance. Technical details for each of these algorithms (except random sorting) as well as a discussion of randomization issues in general are given in Rosenberger and Lachin (2002), Pocock (1983), and Piantadosi (2005). Why Randomize? Random allocation has been an important part of scientific discovery since the time of R.A. Fisher. He and other scientists recognized the potential for bias in nonrandomized experiments that could compromise treatment comparisons. Randomization controls the influence of unknown factors, reducing potential bias in an experiment. Rosenberger and Lachin (2002) state on page 18 that “…the randomization of subjects between the treatment groups is the paramount statistical element that allows one to claim that a study is unbiased.” The elimination of bias, however, is not the only reason for randomization. Pocock (1983) states on page 50 that randomization is used “to provide a basis for the standard methods of statistical analysis such as significance tests.” Clearly, randomization is necessary in order to produce accurate and generalizable statistical conclusions.
[Show full text]
Week 10: Causality with Measured Confounding
Week 10: Causality with Measured Confounding Brandon Stewart1 Princeton November 28 and 30, 2016 1These slides are heavily influenced by Matt Blackwell, Jens Hainmueller, Erin Hartman, Kosuke Imai and Gary King. Stewart (Princeton) Week 10: Measured Confounding November 28 and 30, 2016 1 / 176 Where We've Been and Where We're Going... Last Week I regression diagnostics This Week I Monday: F experimental Ideal F identification with measured confounding I Wednesday: F regression estimation Next Week I identification with unmeasured confounding I instrumental variables Long Run I causality with measured confounding ! unmeasured confounding ! repeated data Questions? Stewart (Princeton) Week 10: Measured Confounding November 28 and 30, 2016 2 / 176 1 The Experimental Ideal 2 Assumption of No Unmeasured Confounding 3 Fun With Censorship 4 Regression Estimators 5 Agnostic Regression 6 Regression and Causality 7 Regression Under Heterogeneous Effects 8 Fun with Visualization, Replication and the NYT 9 Appendix Subclassification Identification under Random Assignment Estimation Under Random Assignment Blocking Stewart (Princeton) Week 10: Measured Confounding November 28 and 30, 2016 3 / 176 1 The Experimental Ideal 2 Assumption of No Unmeasured Confounding 3 Fun With Censorship 4 Regression Estimators 5 Agnostic Regression 6 Regression and Causality 7 Regression Under Heterogeneous Effects 8 Fun with Visualization, Replication and the NYT 9 Appendix Subclassification Identification under Random Assignment Estimation Under Random Assignment Blocking Stewart
[Show full text]
Randomization
Randomization Randomization in trials is the act of assigning treatment units (usually persons) to treatment via a random process. As noted in the essay on random, the expectation is that randomization will produce treatment groups having comparable entry and baseline characteristics. If one wants a "guarantee" of comparability, one has to resort to stratification or matching. Although the expectation is usually satisfied, there is no "guarantee" of comparability. Large differences in the treatment groups can occur by chance in the same way that one can have a fortuitous or disastrous run at the blackjack table. Hence, an aberrantly small p-value for a given baseline variable does not, by itself, provide evidence of a "randomization breakdown". The primary purpose of randomization is to avoid treatment-related selection bias. The notion of bias free assignment is at the heart of any comparative trial and chance via randomization represents the best available tool for achieving bias-free assignment. randomization n - 1. An act of assigning or ordering that is the result of a random process such as that represented by a sequence of numbers in a table of random numbers or a sequence of numbers produced by a random number generator, eg, the assignment of a patient to treatment using a random process. 2. The process of deriving an order or sequence of items, specimens, records, or the like using a random process. rt: haphazardization, quasirandomization Usage note:Donot use as a characterization except in settings where there is an explicit or implied mathematical basis for supporting the usage, as discussed in usage notes for random adj.
[Show full text]
An Introduction to Randomization*
An Introduction to Randomization* TAF – CEGA Impact Evaluation Workshop Day 1 * http://ocw.mit.edu (J-PAL Impact Evaluation Workshop materials) Outline Background What is randomized evaluation? Advantages and limitations of experiments Conclusions An example: The “Vote 2002” campaign Arceneaux, Gerber and Green (2006) Intervention: get-out-the-vote phone calls to increase voter turnout in Iowa and Michigan, 2002 midterm elections Treatment group = 60,000 individuals (35,000 actually reached by phone) Control group = >2,000,000 individuals Main outcome: turnout (did the individual vote?) Effect sizes using experimental v. non-experimental methods How to measure impact? What would have happened in the absence of the intervention program? { Since the counterfactual is not observable, key goal of all impact evaluation methods is to construct of “mimic” the counterfactual Constructing the counterfactual Counterfactual is often constructed by selecting a group not affected by the program Randomized: { Use random assignment of the program to create a control group which mimics the counterfactual. Non-randomized: { Argue that a certain excluded group mimics the counterfactual. Validity A tool to assess credibility of a study Internal validity { Relates to ability to draw causal inference, i.e. can we attribute our impact estimates to the program and not to something else External validity { Relates to ability to generalize to other settings of interest, i.e. can we generalize our impact estimates from this program to other populations,
[Show full text]
Randomized Clinical Trials
15 Randomized clinical trials One pill makes you larger, and one pill makes you small. And the ones your mother gives you, don’t do anything at all. Grace Slick, The Jefferson Airplane: White Rabbit, from Surrealistic Pillow, 1967 Learning objectives In this chapter you will learn: ● the unique features of randomized clinical trials (RCTs) ● how to undertake critical interpretation of RCTs The randomized clinical trial (RCT) is the ultimate paradigm of clinical research. Many consider the RCT to be the most important medical development of the twentieth century, as their results are used to dictate clinical practice. Although these trails are often put on a pedestal, it is important to realize that as with all experiments, there may be flaws in the design, implementation, and interpretation of these trials. The competent reader of the medical literature should be able to evaluate the results of a clinical trial in the context of the potential biases introduced into the research experiment, and determine if it contains any fatal flaws Introduction The clinical trial is a relatively recent development in medical research. Prior to the 1950s, most research was based upon case series or uncontrolled observations. James Lind, a surgeon in the British Navy, can claim credit for performing the first recorded clinical trial. In 1747, aboard the ship Salisbury, he took 12 sailors with scurvy and divided them into six groups of two each. He made sure they were similar in every way except for the treatment they received for scurvy. Dr. Lind found that the two sailors who were given oranges and lemons got better while the other ten did not.
[Show full text]
Randomization Distributions
Section 4.4 Creating Randomization Distributions Statistics: Unlocking the Power of Data Lock5 Randomization Distributions p-values can be calculated by randomization distributions: simulate samples, assuming H0 is true calculate the statistic of interest for each sample find the p-value as the proportion of simulated statistics as extreme as the observed statistic Today we’ll see ways to simulate randomization samples for a variety of situations Statistics: Unlocking the Power of Data Lock5 Cocaine Addiction • In a randomized experiment on treating cocaine addiction, 48 people were randomly assigned to take either Desipramine (a new drug), or Lithium (an existing drug), and then followed to see who relapsed • Question of interest: Is Desipramine better than Lithium at treating cocaine addiction? Statistics: Unlocking the Power of Data Lock5 Cocaine Addiction • What are the null and alternative hypotheses? • What are the possible conclusions? Statistics: Unlocking the Power of Data Lock5 Cocaine Addiction • What are the null and alternative hypotheses? pD, pL: proportion of cocaine addicts who relapse after taking Desipramine or Lithium, respectively ̂ H0: pD = pL H : p < p a D L • What are the possible conclusions? Reject H0; Desipramine is better than Lithium Do not reject H0: We cannot determine from these data whether Desipramine is better than Lithium Statistics: Unlocking the Power of Data Lock5 R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R R 1.
[Show full text]
De-Randomization Under a Uniform Assumption
Randomness vs. Time: De-randomization under a uniform assumption Russell Impagliazzo∗ Avi Wigdersony Department of Computer Science Institute of Computer Science University of California Hebrew University San Diego, CA 91097-0114 Jerusalem, Israel 91904 [email protected] [email protected] Abstract 1 Introduction, History, and Intuition We prove that if BPP = EXP, then every prob- lem in BPP can be solved6 deterministically in 1.1 Motivation subexponential time on almost every input ( The introduction of randomization into efficient on every samplable ensemble for infinitely many computation has been one of the most fertile and input sizes). This is the first derandomiza- useful ideas in computer science. In cryptogra- tion result for BP P based on uniform, non- phy and asynchronous computing, randomization cryptographic hardness assumptions. It implies makes possible tasks that are impossible to per- the following gap in the average-instance com- form deterministically. Even for function com- plexities of problems in BP P : either these com- putation, many examples are known in which plexities are always sub-exponential or they con- randomization allows considerable savings in re- tain arbitrarily large exponential functions. sources like space and time over deterministic al- We use a construction of a small \pseudo- gorithms, or even \only" simplifies them. random" set of strings from a \hard function" But to what extent is this seeming power of in EXP which is identical to that used in the randomness over determinism real? The most analogous non-uniform results of [21, 3]. How- famous concrete version of this question regards ever, previous proofs of correctness assume the the power of BP P , the class of problems solv- \hard function" is not in P=poly.
[Show full text]