(5) designing a study ii (cont.)

7/30/2019 (5) Designing a Study II (Cont.)

1/8

Types of observational studies

1. Cross-sectional studies

2. Prospective cohort studies

3. Case-control studies

4. Nested case-control studies

5. Ecologic studies

Last time we discussed the first two studies, we said the cross sectional

study is done at a single point of time, it takes a short period of time, and

its less expensive.

The other type is called prospective cohort study, prospective means that it

has something to do with the future,so you select your sample and youfollow up the sample for a long period of time.

Slide (19-23): case control study

Subjects are assembled based on whether they have experienced the

o utcome (cases) or not (controls).

In case control studies you take for example patients who have cancer we call them

case, and also you select other normal people we call them controls, then you observe

certain things among these cases and also among these controls.

Frequencies of risk factors are compared between cases and controls .

You select 50 patients who have lung cancer and then you select 50 normal people, and

then you will ask these patients questions about risk factors like smoking, if they

smoke or not, and then you ask the controls normal people if they smoke or not, so you

will study the frequency of the risk factors or the effects of the risk factors which is

smoking on people with lung cancer and on people without lung cancer. So this called

case control study, why Case control? Because one patient has the case and theother one is normal control.

1


2/8

Advantages :

Is Efficient, especially for studying uncommon diseases.

Cases and controls must originate from the same population .

You cant take the people with cancer among the Jordanian and then the

controls from the Chinese people that work in madeenet 2l 7asan!!!

Because they are from different populations.

Disadvantages:

Cannot be used to determine prevalence and incidence.

Because they already have the disease, if you want to see theincidence of cancer, you have to select healthy people and study

them over a long period of time and record new cases of cancer.

Selection bias: loss of cases/controls prior to their selection (a

case died prior to assembly of cases, then the sample wouldnt

be representative)

You want to study people with cancer, and you prepared a research for fifty

cases with cancer, but at the selecting stage, you may discover that some

patients died, so by this you lose some of the cases, and this lead to selection

bias, you need now to select new people, which takes time.

Recall bias

Cases are more likely to remember exposures than

controls,

Psychologically, people with cancer (or any other disease) are different

from normal people, all the time they remember their exposure to cancer,

and that may affect their participation.

E.g., cases with cancer may report previous exposures

because they have been more aware about their health

and subjected to many previous tests.

2


3/8

People with cancer are very cautious about their health, and thats why

they are very aware, thats why they will provide more details about any

previous problem related to cancer that happened with them, but normal

people because they are healthy, they are not caring about the details of

things that happened to them, thats why these two groups are different,

and thats called recall bias, Because people with cancer will remember

more details to provide you in the research more than the normal people

who havent been experienced with cancer.

Can be matched and unmatched

Matching (two types):

1. Individual matching

Each case is individually matched with one or more controls.

For example a 45 yrs old man as case is matched with a 45 yrs old

man as a control.

2. Frequency matching

i. Controls are matched to cases as a group.

We are not matching at individual levels; here we matchone group with the other group

ii. Similar distribution of cases and controls on each

matched variable is established.

iii. E.g., males with range of 20-40 yrs account for 30% in

both groups

If you want in the case group to select ages from twenty

to forty, and this age range account for thirty percent of

the number of cases, you have also to select the same

percentage of people from twenty to forty years in the

control group.

Advantage of matching

You eliminate confounding; we already discussed what confounding

means.

Disadvantage of matching

Increasing difficulty and cost of identifying control, specially with

limited number of potential controls.

3


4/8

its not easy to find two persons with the same circumstances in the

same population, its difficult and can be costly, and in control group,

people are not interested to test a drug on them, we can select fifty

people with cancer and apply a drug, or a medication on them, butthe control group is not easy to select because they will not accept to

try medications on them thats why they have to provide some

source of motivation to them, like paying him with high cost, thats

why it is difficult and expensive.

Matching for a variable will not enable to study its impact on the

outcome

We want to study the effect of diesel fumes in the development of lung

cancer, so what we do? we have to match for smoking, because smoking

also leads to lung cancer, thats why when you select the cases, you haveeither to select cases who are all smokers, or cases who are all non

smokers, you cannot haphazardly go and select people without knowing

that these people are smokers or not.

Best to avoid matching except in small studies where it is difficult to

adjust statistically for all possible confounders unless if matching is

used.

in general although matching is good and proficient, it is difficult and

expensive, so its better to avoid matching, but we do matching

when we have a very small study, which is very easy to handle, like if

you want to study only five cases of cancer.

How many controls per case to enroll?

Greatest efficiency with equal number, like fifty cases and fifty

controls.

Adding additional controls, we can make the control group bigger

than the case group when :

4


5/8

Enough cases cannot be obtained such as in rare conditions to

increase the power of the study.

If we want to study a very rare condition, like a very rare cancer, and

we only have five cases; its not wise to bring only five controls, so

we have to increase the control group.

There is more than one variable or confounder to match on.

If we have more than one variable that can be acting as confounders,

like the effect of smoking on people who breathe diesel fume in the

lung cancer. Smoking is a variable you have to be aware of, thats

why in these cases we have more than one variable. Sometimes the

age is an important variable you have to select people with the sameage in this case you can select more people in the control than in the

case group.

Maximum: 4 controls per case, if you have a case group with twenty

people dont make a control group with more than eighty people.

Slide (24, 25)

Self reading: not included

Slide (26): Specifying a hypothesis

What are you hoping to prove before data collection? Whats your aim in

the research? This is the hypothesis.

Hypothesis, can take two forms:

Null form

There is no difference; or there is no relationship, for example I want

to study the effect of smoking on cancer, I put the null form of my

hypothesis: there is no relationship of smoking on lung cancer, andduring the study I try to reject the null hypothesis, and prove that

smoking causes lung cancer, which is the alternative form.

5


6/8

Alternative form

There is a difference; or there is a relationship

Study hypothesis is stated in both the null and alternative forms, when

we put the hypothesis we put it in two forms, for example I want to

study the effect of smoking on lung cancer, I will have two hypothesis,

the first hypothesis; smoking is not related to cancer this is the null

hypothesis, then smoking is related to lung cancer this Is the

alternative hypothesis, and in my study I want either to reject the null

hypothesis or to confirm the alternative hypothesis (the same meaning),

sometimes the results will be deviated to the null form, and the null

hypothesis become confirmed rather than rejected, and the result will

be there is no relation between smoking and lung cancer in this casethe null hypothesis is not rejected, and you get results that you are not

wishing to, or you are not hoping to.

Statistical analysis is based oninferential reasoning

Slide (27): Inferential reasoning

Assessing the probability that an association found in a sample could haveoccurred bychance if there were no true association in the population.

If the amount of chance in your sample is large and not reduced, some of

your outcomes or findings may be based on chance. In this case - when an

association found by a sample could have occurred by chance- there will be

no true association in the population and your sample will not represent

the population.

Thats why your role is to reduce the amount of chance, so that your

sample represent your population, you dont want the effect of chance tolet you confirm your results when your results cannot be applied on the

population.

If the probability that the association could have occurred by chance falls

below five percent, null hypothesis is rejected and alternative hypothesis is

accepted.

We have a small range of probability of chance, this chance shouldnt be

over five percent, in the study you cannot eliminate chance, because the

sample is not hundred percent representative to the population, but youhave to reduce the chance, I want the chance only to have less than five

percent effect, so thats why I put probability less than five percent, this is

6


7/8

the maximum amount of chance that I can allow in my study otherwise if it

exceeds this percent this means that the outcome is not valid.

We all know that smoking causes lung cancer, but possibly -by chance- I examined a

small group of people in my study and I proved that smoking does not cause lung

cancer, but this is wrong, because it happened by chance and does not represent the

population.

Slide (28): A study with more than one question

Collecting data on more than one outcome

Collecting data on additional risk factors for the same outcome

does not answer multiple questions

If you want to answer more than one question you should have multipleoutcomes, the effect of diesel fume on lung cancer has just one outcome

but smoking may affect the lung cancer, gingival health, and heart disease.

Multiple outcomes

Different stages of the same disease process

Smoking on angina, MI and death

Different disease processes influenced by the same risk factor

Smoking on gingival health, lung cancer and heart disease

Outcomes unrelated to one another

The effect of Smoking on health and cost.

Slide (29): Sample size

Depends on the statistical test used

Discussed later

Slide (30): Ethical approval

Its related or applied to the Human research committees; I cannot do any

research that I want just like that!!!

7


8/8

3. Protect the rights of subjects.

4. Insures that the subjects fully informed.

5. Insures that subjects have consented to participate, like signing a

contract (mwafaqa 5a6eyye)

6. Insures that the risks are reasonable and much less than the new

knowledge or benefit that the study will provide. (fawa2ed lazem

tkuun aktar men 2l 2draar)

7. Insures that confidentiality is maintained, you shouldnt relies the

data you collected, like pictures and radiographs you used in the

research.

One of our colleges in just wanted to make a research on the beauty, and howattractive the face is, he wanted to bring nice females and to have their faces

photographed and then studying it, in our conservative society this is unethical, and

it was rejected.

Thank you LAZGA for helping me SO MUCH in doing this lecture :S!!!(4 minutes!!!)

As we all know from dr.ashraf, most of the question will be from the slides, but to

understand the slides we have to refer to the lectures, the worst thing in studying the

lectures is that there are no pictures AT ALL! :S

Why do we always read the regards before we start the lecture!!! O.o ???

Good luck in the coming exams

DONE BY:

AMMAR ANAGREH

8

THE END

(5) designing a study ii (cont.)

Documents