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DATA COLLECTION AND EXPERIMENTAL DESIGN SECTION 1.3 NOTES.

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Presentation on theme: "DATA COLLECTION AND EXPERIMENTAL DESIGN SECTION 1.3 NOTES."— Presentation transcript:

1 DATA COLLECTION AND EXPERIMENTAL DESIGN SECTION 1.3 NOTES

2 DESIGNING A STATISTICAL STUDY 1.Identify variable of interest and the population of the study 2.Develop a detailed plan for collecting data. If you use a sample, make sure the sample is representative of the population. 3.Collect the data 4.Describe the data, using descriptive statistics techniques. 5.Interpret the data and make decisions about the population using inferential statistics. 6.Identify any possible errors.

3 HOW DO WE COLLECT DATA? 1.Do an observational study.- In observational studies the assignment of elements to different treatments is voluntary, and the experimenter observes the results. Other variables can affect the outcome so we cannot conclude a cause/effect relationship. Sometimes observational studies are the only way. Example: Cell Phone usage linked to brain cancer.

4 HOW DO WE COLLECT DATA? 2. Perform an Experiment- A treatment is applied to part of a population and responses are observed. Another part of the population may be used as a control group, in which no treatment is applied. In many cases, subject (or experimental units) in the control group are given a placebo, which is a harmless, unmedicated treatment.

5 HOW DO WE COLLECT DATA? 3.Use a Simulation- A simulation is the use of a mathematical or physical model to reproduce the conditions of a situation or process. Collecting data often involves the use of computers. Why use a simulation? They allow us to study situations that are impractical or even dangerous and can save time and money.

6 HOW DO WE COLLECT DATA? 4. Use a Survey- A survey is an investigation of one or more characteristics of a population. Most often, surveys are carried out on people by asking them questions.

7 WHICH METHOD IS BEST? 1.A study of the effect of exercise on relieving depression. 2.A study of the success of graduates of a large university in finding a job within one year of graduation.

8 EXPERIMENTAL DESIGN- CRR Control-try to control as many variables as we can. Beware of confounding variables. Does ice cream cause drowning? The placebo effect can also cause confounding. If I think I’m getting treatment I must be getting better. Hawthorne Effect- If I’m in an experiment I must be getting better. Blinding is introduced to decrease these effects. Subjects don’t know if they are in control or experimental group. Double Blind- neither experimenter nor subjects know if the subjects are receiving treatment.

9 RANDOMIZATION The process of randomly assigning subjects to different treatment groups. Why? - Random samples reduce the variability between treatment groups.

10 Completely Randomized Design Example: Random Assignment Treatment 1 Results Treatment 2 Results Compare Control Results

11 Randomized Block Design Example: Random Assignment Treatment 1 Results Block 1 Compare Treatment 2 Results Compare Treatment 1 Results Block 2 Compare Treatment 2 Results

12 REPLICATION The repetition of an experiment under the same or similar conditions.

13 WHAT’S THE PROBLEM? A company identifies ten adults who are heavy smokers. Five of the subjects are given the new gum to quit smoking and the other five are given a placebo. After two months the subjects are evaluated and it is found that the five subjects using the new gum quit. The company then identifies 1000 adults who are heavy smokers. The subjects are divided into blocks according to gender. Females are given the new gum and males are given the placebo. After two months, a significant number of females have quit.

14 WHAT’S THE PROBLEM? Suppose the company identifies 240 adults who are heavy smokers. The subjects are randomly assigned to be in a treatment group or in a control group. Each subject is also given a DVD featuring the dangers of smoking. After four months, most of the subjects in the treatment group have quit smoking.

15 SAMPLING TECHNIQUES Census-a count or measure of an entire population. Sampling-a count or measure of part of a population. The sample should be representative of the population. Why don’t we always take a Census? Too much time and money, or is it even possible?

16 SAMPLING TECHNIQUES A random sample is one in which every member of the population has an equal chance of being selected. A simple random sample is a sample in which every possible sample of the same size has an equal chance of being selected. Random number table, Calculator-random integer How would we choose 10 people at random from a class of 32? Sampling with replacement or sampling without replacement. Can sample contain the same element from the population, or should each element be chosen only once?

17 SAMPLING TECHNIQUES Stratified Sample- A population is broken into different groups or strata, and then samples are taken from each group. Encyclopedias-Certain pages taken from all volumes A through Z Cluster Sample- A population is broken into similar groups(or clusters), and then all elements from one or several of the groups are chosen. Encyclopedias-Certain volumes are chosen, and all pages are taken from those volumes. Systematic Sample- Each member is assigned a number and then sample members are selected at regular intervals. Encyclopedias- Choose every 100 th page in each volume. Convenience Sample- A non-random sampling technique that leads to biased results. Choose the sample that is convenient.

18 Which kind of sampling is it? Modern Managed Hospitals is a national for- profit chain of hospitals. They want to survey patients discharged this past year to obtain patient satisfaction profiles. a. Obtain a list of patients discharged from all MMH hospitals. Divide the patients according to length of stay (2 days or less, 3-7 days, 8-14 days, more than 14 days). Draw simple random samples from each group. b. Obtain lists of patients discharged from all MMH facilities. Number these patients, and then use a random-number table to obtain the sample. c. Randomly select some MMH facilities, and then include all the patients on the discharge lists of the selected hospitals. d. At the beginning of the year, instruct each MMH facility to survey every 500 th patient discharged. e. Instruct each MMH facility to survey 10 discharged patients this week and send in the results.


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