"Case Control Study"........ In 10 Mintues !!!!

welcome to this exciting session of epidemiological studies today what we are going to look at is a type of analytical study design that is case-control in another session we have seen what our cohort studies so today we are going to look at case control study designs the most common and most frequently used analytical study designs in epidemiological studies so when we use the word case control it automatically means that this study has got two groups of people or study subjects one is the cases and another is control now the word case in epidemiology means a person suffering from a given health condition or disease so we use the same term when we design a case control study so when we begin our case control study we need to look at who are going to be our cases and who are going through our controls so the point to remember in a case control study is when we start our study we will have individuals who are diseased as our cases and individuals who are not diseased as our controls so this is the differentiating point between a cohort study and a case control study we start with the disease in a case control study so our two groups are based on presence or absence of the disease so presence of the disease is the cases and absence of the disease is our controls so when we begin a case control study what we do is we first have our cases selected and for every case we will have a control selected now how do we do this let's take an example if I want to do a case control study to study the association between use of oral contraceptives and breast cancer among women who are 40 years and so that's my research question or that's what I want to study so how do I start I start with women who are suffering from breast cancer or who have been diagnosed with breast cancer based on the criteria which I have been Slyke recently diagnosed the grade of cancer and the age group which I want to target so all women who satisfy this criteria or eligibility criteria for my study I am included as cases now who are going to be my controls this is a very important feature of case control studies and the most difficult part I guess because selection of controls is very very important and it is it may decide on how your results are going to be what I mean by that is when you select your controls as I said earlier they should be free from the disease under study so our controls can be women who are over 40 years of age but who do not suffer from breast cancer now the question still remains whether these women who are our controls can they be suffering from any other disease or do they have to be healthy controls so the answer to this is we can either have women who are not suffering from any other disease but we can also include women who are suffering from other cancers or other types of diseases not related to breast cancer this is a very important step so what we do is for every case we'll have to select a control now you can come across a radium material which says that the ratio of cases to controls can be either one is to one that means for one case one control or it could be 1 is to 2 1 is to 4 and so on you can decide on this ratio but the minimum is one case and one control so for every case you need control now if you think of the word bias here we can have selection bias in this particular kind of study if we do not select the cases and the controls appropriately now to eliminate this kind of bias we have to do a process known as matching now what is the meaning of the word matching as I said earlier cases are the ones with disease and controls are the one without disease so the cases and the controls if we match them it means that they can differ from each other only in the way that one suffers from the disease and the other does not suffer from the disease however all the other characteristics life in this case the age group or maybe the occupation all socio-economic characteristics of that person of who is a case matches with that of control what I mean by that is if you have a case which is like a 56 year old woman suffering from grade 1 breast cancer as your case you need to select as a control ideally for matching 56 year old woman who does not suffer from breast cancer maybe belonging to the same socioeconomic status this is what we call as matching now matching can be done for either one characteristic or more than one characteristic in this case I try to match for age so we have women of the same age in both the groups and I also tried to match for socio-economic class so women from the same socio-economic class are included as cases and controls so that's what I mean by matching now what's the next step having selected the cases and controls what we try to do is we try to measure the exposure now in this case what's our exposure use oral contraceptives which has occurred in the past is our exposure all right and no exposure is not having used oral contraceptives in the past so what is important to remember you hear this we start with the disease and we look back for the exposure in the past so case control studies are also retrospective studies the reason being they start with the disease in the present and they look for exposure which is occurred in the past so this is an important thing to remember that we start with the disease and we look out for exposure or we measure the exposure which is occurred in the past now what problems we can encounter if we are trying to recall something which is occurred in the past it exactly results in something which we call as a recall bias some women may have used oral contraceptives in the past but they may quite not remember for how long on what was the duration when they used it on what brand of oral contraceptives they have used depending on what you want to extract from them so this is a very important feature that we need to remember that when we are trying to measure exposure which is occurred in the past we have to rely solely on the information provided by our study subjects so we have to be very careful in what we are trying to get from our study subjects so what we have seen till now is is selected our cases we selected the controls and we have looked in both these groups whether use of oral contraceptives is present in other words exposure is present or there is no use of oral contraceptives which means that there is no exposure now at the end of all this what do we get what will be measured so the measure of disease frequency in a case control study is prevalence because we are starting off with cases of disease which already exist so what we get in a case control study is prevalence of the disease or firstly we can find out proportion of cases of the disease in those who are exposed and proportion of the cases of the disease those who are not exposed so we do not get any incidents in a case control study we just get prevalence because we are starting off with already existing cases of the disease second thing the measure of Association which we calculate in a case control study is known as an odds ratio also known as our and other name for odds ratio is estimated relative risk why we say that is because we just measure the likelihood or the chance of suffering from a disease from the disease and the question in the exposed group and the non exposed group the calculation for all the ratio has already been seen in another session on measuring risk in epidemiology so based on what is the odds of exposure odds of the disease odds of exposure in the disease group and also exposure in the non disease group that's what we are going to compare when we do an odds ratio so in a case control study we get two things a measure of disease frequency which is prevalence and secondly measure of Association which is odds ratio we cannot calculate a relative risk in a case control study the reason being we cannot get incidence in a case control study so that's about the design and the measures which we calculate in a case control study finally let's look at what are the strengths and weaknesses of doing case control studies now case control studies are very very useful when we have limited resources like we need to finish the study in a short time we have less manpower or less money or finances to conduct the study so it is a relatively quicker cheaper and easier type of study design as compared to a old study we can use relative risk we can use case control studies to study rare diseases so it's a very good study design to study rare diseases and we can study multiple exposures when we talk about case control studies the disadvantage of a case control study is if we talk from the point of view of proving causation of a disease the ability of case control studies is not that good when we talk about proving causation the reason being we start with the disease so a temporal sequence which we need to establish to prove causation cannot be found in a case control study secondly we have spoken a bit earlier about the types of bias we can have selection bias and recall bias as the most common disadvantages of case control study so we have to be very careful when we select our cases and controls however the bias of recall we cannot eliminate it completely the reason being the events have occurred in the past and we cannot ensure be assured of that whatever information our study subjects are giving us would be correct or would be reliable so these are the advantages and the disadvantages of case control study so that's all in a nutshell about this very useful analytical study design case control study keep watching keep subscribing and let me know your comments thank you very much have a great day

Channel: Clinical Pharmacology & Public Health

Views: 55,614

Rating: 4.9155889 out of 5

Keywords: Case-control Study, Epidemiology (Medical Specialty), Control Theory (Field Of Study), Medicine (Field Of Study), Preventive, Preventive Medicine (Medical Specialty), Biostatistics (Field Of Study), Cohort study, Study designs

Id: kE81bvv3GCQ

Length: 10min 44sec (644 seconds)

Published: Fri Jul 04 2014