CHM 371 #6
RESEARCH METHODOLOGY
Asst.Prof.Dr. Kanda Wongwailikhit
Objective
Student should be able to...
1. distinguish between an observational study and
a designed experiment
2. Differentiate the sampling method
Observational Studies
Observational Studies
An observational study measures
• the characteristics of a
population by studying individuals in a sample
• does not attempt to
manipulate or influence the variables of interest.
A designed experiment study will
• applies a treatment to
individuals (referred to as experimental units or
subjects)
• attempts to isolate the
effects of the treatment on a response variable.
Designed Experiments
Visit this link from Science Daily, from July 8th, 2008.
This was a tricky one. It was actually an observational study. The key is that the researchers didn't force the veterans to have PTSD, they simply observed the rate of heart disease for those soldiers who have PTSD and the rate for those who do not.
This was a tricky one. It was actually an observational study. The key is that the researchers didn't force the veterans to have PTSD, they simply observed the rate of heart disease for those soldiers who have PTSD and the rate for those who do not.
http://www.sciencedaily.com/releases/2008/07/080707081834.h tm
EXAMPLE 1
EXAMPLE 1
It talks about the relationship between Post-Traumatic Stress Disorder (PTSD) and heart disease.
Visit this link from the ,
EXAMPLE 2
EXAMPLE 2
If you were thinking that this was an observational study, you were right!
The key here is that the individuals sampled were just asked what was important to them. The study didn't try to impose certain conditions on people for a set amount of time and see if those conditions affected their responses.
If you were thinking that this was an observational study, you were right!
The key here is that the individuals sampled were just asked what was important to them. The study didn't try to impose certain conditions on people for a set amount of time and see if those conditions affected their responses.
http://www.gallup.com/poll/10806 7/Fuel-Prices-Now-Clearly-America ns-No-Concern.aspx
It looks at what Americans' top concerns were at that point. Do you think this was an observational study or a designed experiment? Why?
This last example is regarding the "low-carb" Atkins diet, and how it compares with other diets.
Read through this summary of a report in the New England J. of Med.
Can you figure out whether it's …
(a) observational study
(b) a designed experiment EXAMPLE 3
EXAMPLE 3
As expected, this was a designed experiment, but do you know why? The key here is they forced individuals to maintain a
certain diet, and then compared the participants' health at the end.
As expected, this was a designed experiment, but do you know why? The key here is they forced individuals to maintain a
certain diet, and then compared the participants' health at the end.
There are 3 major types of observational studies
1. cross-sectional studies,
2. case-control studies
3. cohort studies.
Types of Observational Studies
Cross-sectional Studies
Cross-sectional studies don't give very strong results. In above
example, we can't be sure that those working with asbestos who don't report cancer won't eventually develop it.
This type of study only gives a bit of the picture, so it is rarely used by itself. Researchers tend to use a cross-sectional study to first
determine if their might be a link, and then later do another study (like one of the following) to further investigate.
Cross-sectional studies don't give very strong results. In above
example, we can't be sure that those working with asbestos who don't report cancer won't eventually develop it.
This type of study only gives a bit of the picture, so it is rarely used by itself. Researchers tend to use a cross-sectional study to first
determine if their might be a link, and then later do another study (like one of the following) to further investigate.
study involves collecting data about
individuals at a certain point in time.
A researcher concerned about the effect of working with asbestos might compare the cancer rate of those who work with asbestos versus those who do not.
EX
Cross-sectional Studies scheme
Case-control Studies
Case-control studies are frequently used in the medical community to compare
individuals with a particular characteristic (this group is the case)with individuals who do not have that characteristic (this group is the control).
A control group is a group in an experiment
that is under normal conditions. If one was
testing out fertilizer, the control group would
be a plant with just normal dirt.
Case-control Studies (cont.)
One of the most famous examples of this type of study is the “ link between smoking and cancer”.
almost 80% of adults in the UK were smokers, and the connection between smoking and lung cancer had not yet been established.
Doll and Hill interviewed about 700 lung cancer patients to try to determine a possible cause.
2
This type of study is retrospective, because it asks the individuals to look back and describe their
Case-control Studies scheme
Cohort Studies
Cohort Studies
A cohort describes a group of individuals, and so a cohort study is one in which a group of individuals is selected to participate in a study. The group is then observed over a period of time to determine if
particular characteristics affect a response variable.
Based on their earlier research, Doll and Hill began one of the largest cohort studies in 1951. The study was again regarding the link
between smoking and lung cancer. The study began with 34,439 male British doctors, and followed them for over 50 years. Doll and Hill first reported finding in 1954 in British Med. J., and then
continued to report their findings periodically afterward. Their last report was in 2004, in British Med. J., . This last report reflected on 50 years of observational data from the cohort.
This last type of study is called
prospective
,
because it begins with the group and then
collects data over time.
cohort studies
are definitely
the most powerful
of the observational studies, particularly with
the quantity and quality of data in a study like
the previous one.
Cohort Studies (cont.,)
Cohort Studies scheme
A study concerning dementia and "mid-life ills".
Researches followed more than 11,000 people over a period of 12-14 years. They found that smoking,
diabetes, and high blood pressure were all factors in the onset of dementia.
EXAMPLE 4 EXAMPLE 4
What type of observational study was this? (a) Cross-sectional
(b) case-control (c) cohort?
In 1993, the National Institute of Environmental Health
Sciences funded a study regarding the possible relationship between radon levels and the incidence of cancer.
The study gathered information from 413 participants who had developed lung cancer and compared those results
with 614 participants who did not have lung cancer.
This study was retrospective - gathering information about the group of interest (those with cancer) and comparing them with a control group(those without cancer). This is an example of a case-control study.
EXAMPLE 5 EXAMPLE 5
Thought his may seem similar to a
cross-sectional study, it differs in that the individuals
are "matched" (with cancer vs. without
cancer)and the individuals are expected to look
back in time and describe their time spent in the
home to determine their radon exposure.
In 2004, researchers published an article
regarding the relationship between the mental
health of soldiers exposed to combat stress. .
EXAMPLE 6
EXAMPLE 6
Since this was simply a survey given over a short period of time to try to examine the effect of combat duty, this was a cross-sectional study. Unlike the previous example, it did not ask the participants to delve into their history, nor did it explicitly
"match" soldiers with a particular characteristic.
What type of study was this? (a) Cross sectional study (b) Case control study (c) Cohort study
The study collected
information from soldiers in four combat infantry units either before their
The goal of the cross-sectional study is a better
understanding of the disease and risk factors that are associated with the disease.
The retrospective study is good to get an estimation of the potential risk factors and usually these relationships are confirmed by the prospective studies.
Therefore, the retrospective study is less valid than the prospective method.
The results from these studies have been utilized for
Types of sampling
•
Non-probability samples
•
Probability samples
Probability samples are the best
Ensure
– Representativeness – Precision
Probability of being chosen is unknown Cheaper- but unable to generalise
Non probability samples
Convenience samples
Snowball sampling
Purposive sampling
Convenience samples
(ease of access)
sample is selected from elements of a population
that are easily accessible
•
Man a street interview
•
Teacher use students
•
Asking the one you meet at a supermarket
•
Volunteers
Snowball sampling
(friend of friend….etc.)
Quota sample
• Researcher uses some knowledge of the
population to build some representativeness into the sampling plan
• Divides population into different strata and
samples from each of them
• Usually better than just convenient
Purposive sampling
(judgemental)
•
You chose who you think should be in
the study
QUOTA sampling
Methods used in probability samples
Simple random sampling
Stratified sampling
Systematic sampling
Cluster sampling
Simple Random Sampling
Simple Random Sampling
A sample of size n from a population of size N is obtained through simple random sampling if every possible
sample of size n has an equally likely chance of occurring.
write everyone's name on a slip of paper and draw two from a hat
http://www.youtube.com/watch?v=yx5KZi5QArQ
http://www.youtube.com/watch?v=YamE4KqCv3k
Randomized Controlled Trial
Simple random sampling
Using excell
=RANDBETWEEN(1,24)
=RANDBETWEEN(1,24)
=VLOOKUP(B3,E$2:F$26,2)
HOME WORK for 6
thclass
Use excell program for simple randomized
name of xxxxxx(your patients, school) etc.,
Population > 50 name
(List of name your self)
Sampling for 25% of population
Send to kanda@rsu.ac.th
Email subject : CHM371 random-student ID
HOME WORK for 6
thclass
Use excell program for simple randomized
name of xxxxxx(your patients, school) etc.,
Population > 50 name
(List of name your self)
Sampling for 25% of population
Send to
kanda@rsu.ac.th
Stratified Sampling
Stratified Sampling
Stratified sampling is different. With this technique, we separate the population using some characteristic, and then take a proportional random sample from each.
A stratified sample is obtained by separating the population into non-overlapping groups
Stratified Sampling
One easy example using a stratified technique
would be a sampling of people at ECC.
To make sure that a sufficient number of
students, faculty, and staff are selected, we
would stratify all individuals by their status -
students, faculty, or staff. (These are
the
strata
.)
Then, a proportional number of individuals
would be selected from each group.
Example 1
Systematic Sampling
Systematic Sampling
For our starting point, we pick a random number between 1 and k. For our visual, let's suppose that we pick 2. The individuals sampled would then be 2, 5, 8, and 11.
Systematic Sampling
Systematic Sampling
Systematic sampling works well when the individuals are already lined up in order.
In the past, students have often used this method when asked to survey a random sample of students.
Since we don't have access to the complete list, just stand at a corner and pick every 10th* person walking by.
Systematic Sampling (cont.,)
Cluster Sampling
Cluster Sampling
Cluster sampling is often confused with stratified sampling, because they both involve "groups". In
Begin with names of population
Insert code the characteristic Select column data sort chose sort by
OK
Excell program for systematic sampling
Begin with names
Calculate interval from N/n and round off to integer
Set chosen number to begin
First + N/n
Cluster Sampling
Cluster Sampling
A cluster sample is
obtained by selecting all individuals within a
randomly selected
collection or group of individuals.
In essence, we use cluster sampling when our
Cluster sampling
Section 4
Section 5 Section 3 Section 2
One situation where cluster sampling would apply might be in manufacturing.
Suppose a company makes light bulbs, and would like to test the effectiveness of the packaging. You don't have a
complete list, so simple random sampling doesn't apply, and the bulbs are already in boxes, so you can't order them to use systematic.
And all the bulbs are essentially the same, so there aren't any characteristics with which to stratify them.
To use cluster sampling, a quality control inspector might select a certain number of entire boxes of bulbs and test each bulb within those boxes. In this case, the boxes are the clusters.
Example 3
Multistage Sampling
Multistage Sampling
Often one technique isn't possible, so many professional polling agencies use a technique
called multistage sampling. The strategy is relatively
self-explanatory - two or more sampling techniques are used.
Stage 1: A quality control engineer removes every 200th box coming off the line. (The plant produces 5,000 boxes daily. (This is systematic sampling.)
Stage 2: From each box, the engineer then samples three packages to inspect. (This is an example
of clustersampling.)
Let's suppose that the bulbs come off the assembly line in boxes that each contain 20 packages of four
bulbs each. One strategy would be to do the sample in two stages: