Collecting Data Marc H. Mehlman marcmehlman@yahoo.com University - - PowerPoint PPT Presentation

Marc Mehlman

Collecting Data

Marc H. Mehlman

marcmehlman@yahoo.com

University of New Haven

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Marc Mehlman

Table of Contents

1

Design of Experiment, DOE

2

Sampling Design

3

Inference

4

Ethics

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In contrast to observational studies, experiments don’t just observe individuals or ask them questions. They actively impose some treatment in order to measure the response.

Observation vs. Experiment

An observational study observes individuals and measures variables of interest but does not attempt to influence the

• responses. The purpose is to describe some group or situation.

An experiment deliberately imposes some treatment on individuals to measure their responses. The purpose is to study whether the treatment causes a change in the response. An observational study observes individuals and measures variables of interest but does not attempt to influence the

• responses. The purpose is to describe some group or situation.

An experiment deliberately imposes some treatment on individuals to measure their responses. The purpose is to study whether the treatment causes a change in the response. When our goal is to understand cause and effect, experiments are the

• nly source of fully convincing data.

The distinction between observational study and experiment is one of the most important in statistics.

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Confounding

Observational studies of the effect of one variable on another often fail because of confounding between the explanatory variable and one or more lurking variables. A lurking variable is a variable that is not among the explanatory or response variables in a study but that may influence the response variable. Confounding occurs when two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other. A lurking variable is a variable that is not among the explanatory or response variables in a study but that may influence the response variable. Confounding occurs when two variables are associated in such a way that their effects on a response variable cannot be distinguished from each other. Well-designed experiments take steps to avoid confounding.

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Design of Experiment, DOE

Design of Experiment, DOE

Design of Experiment, DOE

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Design of Experiment, DOE

Definition The individuals on which the experiment is done are the experimental

• units. When the units are human beings, they are called subjects. The

explanatory variables of the experiment are called factors. Often factors are administrated at different levels, ie, strengths. A specific experimental condition (combination of levels from the different factors) applied to a subset of the units is called a treatment. If there exists a collection of experimental units that are not exposed to any of the factors, that group is called the control group. Example Scientists are interested in the effects of two factors in crop yield, namely irrigation and fertilizer. They consider four levels of irrigation; none, once a week, three times a week and daily. They consider three levels of fertilizer, none, moderate application and heavy application. By combining all levels, the scientists come up with twelve treatments. They then apply each treatment to nine of 108 plots of crop land.

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Design of Experiment, DOE

When doing medical experiments using humans, it was often noticed that subjects who knew they were in a control group and hence receiving no treatment did worse than those who were receiving treatment even when the treatment was known to be ineffective! This phenomenon is called the placebo effect, named after the method frequently used to deal with this

• effect. Subjects who do not take pills in a medical study know they are not

getting treatment, so all subjects are given identically looking pills, only some subjects are given placebos, sugar bills with no medical effects. Subjects are not told which treatment they are getting. This does not remove the placebo effect from the experiment, but the placebo effect will now be the same for all treatments. The power of suggestion, ie the placebo effect, is quite strong in humans. A mother’s kiss on her child’s seemly critical wound can often render the scratch painless. Similarly placebos can have positive effects with adults suffering from more serious maladies.

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Design of Experiment, DOE

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Randomized Comparative Experiments

The remedy for confounding is to perform a comparative experiment in which some units receive one treatment and similar units receive

• another. Most well-designed experiments compare two or more

treatments. Comparison alone isn’t enough. If the treatments are given to groups that differ greatly, bias will result. The solution to the problem of bias is random assignment. In an experiment, random assignment means that experimental units are assigned to treatments at random, that is, using some sort of chance process. In an experiment, random assignment means that experimental units are assigned to treatments at random, that is, using some sort of chance process.

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Design of Experiment, DOE

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In a completely randomized design, the treatments are assigned to all the experimental units completely by chance. Some experiments may include a control group that receives an inactive treatment or an existing baseline treatment. In a completely randomized design, the treatments are assigned to all the experimental units completely by chance. Some experiments may include a control group that receives an inactive treatment or an existing baseline treatment.

Experimental Units Experimental Units

Random Assignme nt Random Assignme nt

Group 1 Group 1 Group 2 Group 2

Randomized Comparative Experiments

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Design of Experiment, DOE

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Principles of Experimental Design

Randomized comparative experiments are designed to give good evidence that differences in the treatments actually cause the differences we see in the response.

• 1. Control for lurking variables that might affect the response, most

simply by comparing two or more treatments.

• 2. Randomize: Use chance to assign experimental units to

treatments.

• 3. Replication: Use enough experimental units in each group to

reduce chance variation in the results.

• 1. Control for lurking variables that might affect the response, most

simply by comparing two or more treatments.

• 2. Randomize: Use chance to assign experimental units to

treatments.

• 3. Replication: Use enough experimental units in each group to

reduce chance variation in the results. Principles of Experimental Design Principles of Experimental Design An observed effect so large that it would rarely occur by chance is called statistically significant. A statistically significant association in data from a well-designed experiment does imply causation. An observed effect so large that it would rarely occur by chance is called statistically significant. A statistically significant association in data from a well-designed experiment does imply causation.

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Design of Experiment, DOE

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Cautions About Experimentation

The logic of a randomized comparative experiment depends on our ability to treat all the subjects the same in every way except for the actual treatments being compared. In a double-blind experiment, neither the subjects nor those who interact with them and measure the response variable know which treatment a subject received. In a double-blind experiment, neither the subjects nor those who interact with them and measure the response variable know which treatment a subject received. The most serious potential weakness of experiments is lack of

• realism. The subjects or treatments or setting of an experiment may

not realistically duplicate the conditions we really want to study.

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Design of Experiment, DOE

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Matched Pairs

A common type of randomized block design for comparing two treatments is a matched pairs design. The idea is to create blocks by matching pairs of similar experimental units. A matched pairs design is a randomized blocked experiment in which each block consists of a matching pair of similar experimental units. Chance is used to determine which unit in each pair gets each treatment. Sometimes, a “pair” in a matched pairs design consists of a single unit that receives both treatments. Since the order of the treatments can influence the response, chance is used to determine which treatment is applied first for each unit. A matched pairs design is a randomized blocked experiment in which each block consists of a matching pair of similar experimental units. Chance is used to determine which unit in each pair gets each treatment. Sometimes, a “pair” in a matched pairs design consists of a single unit that receives both treatments. Since the order of the treatments can influence the response, chance is used to determine which treatment is applied first for each unit.

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Design of Experiment, DOE

“Block what you can and randomize what you cannot.”

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Blocked Designs

Completely randomized designs are the simplest statistical designs for

• experiments. But just as with sampling, there are times when the

simplest method doesn’t yield the most precise results. A block is a group of experimental units that are known before the experiment to be similar in some way that is expected to affect the response to the treatments. In a block design, the random assignment of experimental units to treatments is carried out separately within each block. A block is a group of experimental units that are known before the experiment to be similar in some way that is expected to affect the response to the treatments. In a block design, the random assignment of experimental units to treatments is carried out separately within each block.

Form blocks based on the most important unavoidable sources of variability (lurking variables) among the experimental units. Randomization will average out the effects of the remaining lurking variables and allow an unbiased comparison of the treatments. Control what you can, block on what you can’t control, and randomize to create comparable groups. Marc Mehlman (University of New Haven) Collecting Data 13 / 28

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Sampling Design

Sampling Design

Sampling Design

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Sampling Design

The distinction between population and sample is basic to statistics. To make sense of any sample result, you must know what population the sample represents.

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Population and Sample

The population in a statistical study is the entire group of individuals about which we want information. A sample is the part of the population from which we actually collect information. We use information from a sample to draw conclusions about the entire population. The population in a statistical study is the entire group of individuals about which we want information. A sample is the part of the population from which we actually collect information. We use information from a sample to draw conclusions about the entire population.

Population Population Sample Sample

Collect data from a representative Sample... Make an Inference about the Population.

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Sampling Design

Definition A simple random sample, SRS, of size n consists of n individuals from the population chosen in such a way that every set of n individuals has an equal chance to be the sample actually chosen. Simple random samples come from sampling without replacement. Sampling with replacement gives: Definition A random sample of size n individuals results from randomly choosing an individual the population n times, with each choice independent of the

• ther choices.

Example Suppose you are dealt five cards from the top of a shuffled deck. This would be a simple random sample. On the other hand, if you are dealt the top card from five shuffled decks you would have a random sample. The random sample could contain two or more cards of the same type!

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Sampling Design

While theory of simple random samples is not at all simple compared to random samples, in real life simple random samples seem much more

• natural. However one ignores the distinction between simple random

samples and random samples when: When Simple Random Sampling is equivalent to Random Sampling When a population is 100 or more times larger than the sample size, there is very little difference between simple random samples and random samples since the probability the same individual would be chosen twice is extremely small. Sometimes there are statistical advantages to using a more complex sampling procedure. Definition To select a stratified random sample, first classified the population into similar groups of similar individuals, called strata. Then chose a separate random sample in each stratum and combine these random samples to form a full sample.

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Sampling Design

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Cautions About Sample Surveys

Good sampling technique includes the art of reducing all sources of error. Undercoverage occurs when some groups in the population are left out of the process of choosing the sample. Nonresponse occurs when an individual chosen for the sample can’t be contacted or refuses to participate. A systematic pattern of incorrect responses in a sample survey leads to response bias. The wording of questions is the most important influence on the answers given to a sample survey. Undercoverage occurs when some groups in the population are left out of the process of choosing the sample. Nonresponse occurs when an individual chosen for the sample can’t be contacted or refuses to participate. A systematic pattern of incorrect responses in a sample survey leads to response bias. The wording of questions is the most important influence on the answers given to a sample survey.

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Inference

Inference

Inference

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Inference

As we begin to use sample data to draw conclusions about a wider population, we must be clear about whether a number describes a sample or a population.

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Parameters and Statistics

A parameter is a number that describes some characteristic of the

• population. In statistical practice, the value of a parameter is not

known because we cannot examine the entire population. A statistic is a number that describes some characteristic of a

• sample. The value of a statistic can be computed directly from the

sample data. We often use a statistic to estimate an unknown parameter. A parameter is a number that describes some characteristic of the

• population. In statistical practice, the value of a parameter is not

known because we cannot examine the entire population. A statistic is a number that describes some characteristic of a

• sample. The value of a statistic can be computed directly from the

sample data. We often use a statistic to estimate an unknown parameter. Remember s and p: statistics come from samples and parameters come from populations. We write µ (the Greek letter mu) for the population mean and σ for the population standard deviation. We write (x-bar) for the sample mean and s for the sample standard deviation. x

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Inference

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Statistical Estimation

The process of statistical inference involves using information from a sample to draw conclusions about a wider population. Different random samples yield different statistics. We need to be able to describe the sampling distribution of possible statistic values in order to perform statistical inference. We can think of a statistic as a random variable because it takes numerical values that describe the outcomes of the random sampling process. Therefore, we can examine its probability distribution using what we learned in earlier chapters. Population Population Sample Sample Collect data from a representative Sample... Make an Inference about the Population.

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Inference

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Sampling Variability

This basic fact is called sampling variability: the value of a statistic varies in repeated random sampling. To make sense of sampling variability, we ask, “What would happen if we took many samples?” Population Population

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Inference

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Sampling Distributions

If we measure enough subjects, the statistic will eventually get very close to the unknown parameter. If we took every one of the possible samples of a certain size, calculated the sample mean for each, and graphed all of those values, we’d have a sampling distribution. The population distribution of a variable is the distribution of values of the variable among all individuals in the population. The sampling distribution of a statistic is the distribution of values taken by the statistic in all possible samples of the same size from the same population. The population distribution of a variable is the distribution of values of the variable among all individuals in the population. The sampling distribution of a statistic is the distribution of values taken by the statistic in all possible samples of the same size from the same population. In practice, it’s difficult to take all possible samples of size n to obtain the actual sampling distribution of a statistic. Instead, we can use simulation to imitate the process of taking many, many samples.

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Inference

We can think of the true value of the population parameter as the bull’s-eye on a target and of the sample statistic as an arrow fired at the target. Both bias and variability describe what happens when we take many shots at the target. Bias concerns the center of the sampling distribution. A statistic used to estimate a parameter is unbiased if the mean of its sampling distribution is equal to the true value of the parameter being estimated. The variability of a statistic is described by the spread of its sampling distribution. This spread is determined by the sampling design and the sample size n. Statistics from larger probability samples have smaller spreads.

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Bias and Variability

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Inference

A good sampling scheme must have both small bias and small variability. To reduce bias, use random sampling. To reduce variability of a statistic from an SRS, use a larger sample. To reduce bias, use random sampling. To reduce variability of a statistic from an SRS, use a larger sample.

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Managing Bias and Variability

The variability of a statistic from a random sample does not depend

• n the size of the population, as long as the population is at least 100

times larger than the sample. The variability of a statistic from a random sample does not depend

• n the size of the population, as long as the population is at least 100

times larger than the sample.

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Ethics

Ethics

Ethics

Never borrow money from a friend or stranger without notifying them, unless it is for something very important. Never administer too many genetically altering drugs to the neighbor’s dog unless it is for the sake of science. – Professor Marc Mehlman “I am greatly envious of Marc Mehlman’s morals. If I had them, I could do so many more things.” – Professor Wayne Smith, 1970’s

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Ethics

Basic Data Ethics

The most complex issues of data ethics arise when we collect data from people. Basic Data Ethics The organization that carries out the study must have an institutional review board that reviews all planned studies in advance in order to protect the subjects from possible harm. All individuals who are subjects in a study must give their informed consent before data are collected. All individual data must be kept confidential. Only statistical summaries for groups of subjects may be made public. Basic Data Ethics The organization that carries out the study must have an institutional review board that reviews all planned studies in advance in order to protect the subjects from possible harm. All individuals who are subjects in a study must give their informed consent before data are collected. All individual data must be kept confidential. Only statistical summaries for groups of subjects may be made public.

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Ethics

Clinical Trials

Clinical trials study the effectiveness of medical treatments on actual patients—these treatments can harm as well as heal. Points for a discussion: Randomized comparative experiments are the only way to see the true effects of new treatments. Most benefits of clinical trials go to future patients. We must balance future benefits against present risks. The interests of the subject must always prevail over the interests

• f science and society.

In the 1930s, the Public Health Service Tuskegee study recruited 399 poor blacks with syphilis and 201 without the disease in order to

• bserve how syphilis progressed without treatment. The Public

Health Service prevented any treatment until word leaked out and forced an end to the study in the 1970s.

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Note: Racism and unethical experimental design is “confounded” in above example.

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