Measures of Association: Odds Ratio

Public health and measures of association

Establishing association between factors and outcome is one of the most important responsibilities of the epidemiologists and health professionals. We established the association of obesity and cardio-vascular diseases, smoking and lung cancer, alcohol and cirrhosis or liver diseases, junk foods and the obesity, vitamin c and scurvy etc. These are just the handful of examples and many relationships are yet to discover. Here are some of the quantitative measures for establishing association.

1. Relative risk/Risk ratio
2. Odds ratio
3. Risk difference
4. Attributable risk
5. Population attributable risk

What is Odds Ratio?

Here, I would like to explore into odds ratio (OR). Odds ratio is one of the measures of association and is frequently used by public health professionals throughout their professional career. However, we are pretty confused about interpretation of odds ratio sometimes in our professional training or career. Sometimes we interpret it as relative risk, sometimes we would like to simplify it in common words as the probability of happening something, which twist the essence of its meaning in other direction.

Example

Let's delve into calculation of odds ratio through our hypothetical case control study. We would like to assess if hormone replacement therapy affects the occurrence of breast cancer in women. In this study we have two comparison groups: case and control.
Cases and Controls: In this case, Cases would be women with breast cancer and controls would be individually or frequency matched women with no breast cancer.
In order to assess the effect of hormone replacement therapy on breast cancer, we ask their history of past or current hormone replacement therapy depending on the objective of the study. We can visualize the following table with their information on breast cancer and hormone replacement therapy. Here, the hormone replacement therapy is the exposure or the factor of our interest.

Odds ratio is basically the ratio of two odds. In this example, OR is ratio of odds that cases were exposed to hormone replacement therapy and odds that controls were exposed to hormone replacement therapy.

Calculation of odds ratio (OR)

There are two things to understand in calculation of odds ratio. First we need to understand about odds. Odds is the ratio of probability of happening of one event in a group to the probability of not happening of that event in the same group so, odds ratio is simply the ratio of two odds. In odds ratio, we take ratio of those odds in two different groups.

Step 1: At first, we calculate the odds in those comparison groups.

Cases:
$$\text{Odds that a case was exposed}=\frac{\text{Probability of exposures in cases (a)}}{\text{Probability of non-exposures in cases (c)}}$$
$$=\frac{\text{Number of exposed among cases (a)}/\text{total number of cases(a+c)}}{\text{Number of not exposed among cases(c)}/\text{total number of cases (a+c)}}$$
$$=\frac{\text{a}}{/}$$

Controls:
$$\text{Odds that a control was exposed}=\frac{\text{Probability of exposures in controls}}{\text{Probability of non-exposures in controls}}$$
$$=\frac{\text{Number of exposed among controls (b)}/\text{total number of controls(b+d)}}{\text{Number of not exposed among controls(d)}/\text{total number of controls (b+d)}}$$
$$=\frac{\text{b}}{/}$$

Step 2: Finally, we calculate the ratios of these two odds and interpret the differential effect of exposures in those two groups.
$$\text{OR}=\frac{\text{a}/\text{c}}{/}=\frac{\text{ad}}{\text{bc}}$$

Interpretation of Odds Ratio

As we already mentioned elsewhere, Odds ratio is utilized to assess the strengths of association between exposure and the outcome. In our hypothetical illustration, we can see we utilized odds ratio to find the strength of association between exposure as hormone replacement therapy on breast cancer (outcome).
If the odds ratio is more than 1 then the odds of having exposed to hormone replacement therapy is higher in cases than in controls which indirectly implies hormone replacement therapy as a risk factor for breast cancer provided that it fulfills other criteria.



In this study Odds ratio is 9.3 which shows that the odds of having exposed to hormone replecement therapy is 9 times higher for cases compared to controls. Odds ratios are not straight forward like percentage or relative risk but it has close approximation to them.

By Pramila Rai

How to choose a topic for thesis

Thesis is often required for completion of Bachelors and Masters degree. Selecting a topic for thesis seems challenging for every one. Many students go through this bewildered stage once in their study period. Every graduate accept this fact that choosing a topic for thesis is really difficult and selecting a topic is considered thesis half done.

I have met some people, who weren’t satisfied to the topics they selected whereas some rejoiced their thesis they undertook and used it as base of their career as well. So, based on professional experience and the people’s experience, I would like to share some tips for selecting a topic. You all might have heard about the criteria like the research question should be novel, feasible, and blah blah. However I would like to focus on some major features, then you need to look for all those criteria.

1. Career Goals: While selecting a course for you, you must have some career goals in the background of your mind. You must have it somewhere; some might be open about it; some might keep it silent. If you don’t have it, it is certainly better to have one because you will work and follow the career path to reach your career goal. If you have that career goal in your mind, It will be better to select such research topics related to your career goal. If I want to work as employee of UNFPA at some point, it is better to select the related topics like population control measures such as family planning measures, fertility. This will certainly assist you in increasing your credibility and improving CV for your future career goal.

2. Interest: Thesis cost your bulk of time. You search numerous relevant literatures; you delve into its details. You need to invest your time, concentration, mind, thoughts, labour, and money on it so it’s very important to have interest on that topic or the subject and the population you are studying. You should have love for that topic. Your heart should be there, so you will enjoy, when you work on it. If you are detached from the subject of study and if you don’t have passion for it, it will become burden to you. You might learn the basics but you wouldn’t enjoy it and wouldn’t get the best of it.

Nevertheless, the very interested topic might be uninteresting at some point and some starts some topic without any interests but later on develop interest as they work on it. It remains highly subjective and might fluctuate but you also need determination to make you interested all the time.

3. Research Methods: Some of you might be interested to study such topics, which is fit for particular methods. You might be interested to do randomized clinical trial or case control study or cohort study. You could be interested to learn such methods rather than particular issue. You must keep this into your thought as well.

4. Scope: sometimes its better to flow to the direction, where the air is flowing. You will easily catch something on the way. You can also choose the specific topic, which has great scope, which has many projects around. For example, Non-communicable diseases are rising in epidemic fashion and have become issue of global importance in both developing and developed countries; similarly, emerging infectious diseases, population health also have great scope. In some area, though the scope is higher, it can be competitive somehow. All of these things should be kept in the mind.

A thesis mightn’t decide your future and your career path, but will certainly be a pillar in forming your career. You can use this as your strength and your experience. There are people, who change their area of specialization; anyone is free to do it. However, you can make thesis your ladder for future so need to focus on your future prospects too. Based on above mentioned factors, you can select an area of interest and while formulating research question; you need to look for other criteria such as FINER. The research question should be feasible in terms of sample size, time and money, technical expertise, Interesting to the scientific community, Novel: should provide the results that confirms, refutes or extends previous findings, Ethical: must adhere to the principle of ethics, Relevant to the scientific community, future research, current practice and policies.

Verbal Autopsy in Health Research

Many Researchers have been using verbal autopsies method to estimate the number of deaths in the community and to determine underlying causation of death.

Some examples of the study that have adopted Verbal Autopsy:

1. NEPAL Maternal Mortality and Morbidity Study 2008/2009 had used verbal autopsy method to find out the underlying causes of mortality and morbidity.
2. Khanal S, Gc VS, Dawson P, Houston R. Verbal autopsy to ascertain causes of neonatal deaths in a community setting: a study from Morang, Nepal. JNMA J Nepal Med Assoc. 2011 Jan-Mar;51(181):21-7.
3. Stefania Vergnano, Edward Fottrell, David Osrin, Peter N Kazembe, Charles Mwansambo, Dharma S Manandhar, Stephan P Munjanja, Peter Byass, Sonia Lewycka, and Anthony Costello. Adaptation of a probabilistic method (InterVA) of verbal autopsy to improve the interpretation of cause of stillbirth and neonatal death in Malawi, Nepal, and Zimbabwe. Population Health Metrics. 2011; 9: 48.

 

 Verbal autopsy can be used as a complementary method in a research and as a sole method for identifying the death and its causes. In the first aforementioned study of Maternal mortality and Morbidity, verbal autopsy has been used as a complementary method to find the maternal mortality and in the second study verbal autopsy is the solitary method to find the cause of death among neonatal deaths.

What is verbal Autopsy (VA)?

AUTOPSY/POST MORTEM/NECROPSY: Scared!!!! Here a certified doctor performs examination on dead body to find out the causes of death. The purpose of Autopsy and verbal autopsy is same, however, the process is entirely different. Trained medical professionals as well as non-medical professionals can perform verbal Autopsy. However, both of the groups need training for conducting verbal autopsy procedure beforehand.

Verbal Autopsy (VA) is an indirect method of ascertaining biomedical causes of death from information on symptoms, signs and circumstances preceding death, obtained from the deceased’s caretakers. Usually the person, who looked after the deceased during the final illness are asked the details of the dead person.  However, the process of identifying an appropriate respondent is not formalized. Few studies reported interviewing friends or neighbors if a caretaker was not available.

The accuracy of the information obtained through verbal autopsy is essentially affected by the skills of the interviewers, respondents, recall period, languages (required for translation of questionnaires).  In most of research or process, the interviewers are health care professionals, including doctors, nurses and paramedics whereas some employ non-medical professionals. It needs training prior to field work. At a time people might suffer emotional break out when they talk about deceased one, making counseling training and techniques essential part of verbal autopsy process though commonly not practiced.

Timing: Even though verbal autopsies are done as soon as possible after the death to indefinite period.  Most has agreed that a minimum of  4 weeks is required to reduce the distress and emotional disturbance of the respondents. The maximum recall period can vary from six months to an indefinite amount of time. Nevertheless, The period following one month to 12 month is generally considered acceptable.

There is a standard tool for verbal autopsy. In 2007, needs and demands for standardization led to the 2007 publication of the WHO VA standards, which many researchers have adopted. The standards included:

• Verbal autopsy questionnaires for three age groups (under four weeks; four weeks to 14 years; and 15 years and above); 
• Cause of death certification and coding resources consistent with the International Classification of Diseases and Related Health Problems, tenth revision (ICD-10); and 
• A cause-of-death list for verbal autopsy prepared according to the ICD-10.

Why we are employing Verbal Autopsy?

Ideal circumstances: The standard method to determine the cause of death is certification by an attending physician, based on valid medical documents. There should have been updated vital registration system (includes birth certification, marriage certification, death certification and adequate records, migration)

Reality: we don’t have well functioning vital registration system due to many reasons. I would like to focus on death registration: low level of awareness about its relevance, lack of well established vital registration system, limitation in proper diagnosis, limited qualified health care professionals, Rural settings, which is devoid of all of these and most death occur at home, the lack of infrastructure and the high cost of collecting the data, which limit access to information from diagnostic tests and post-mortem pathology services. Mortality data from developing countries are therefore limited and potentially biased.

Why do we need the information regarding the cause of death?

Have you ever thought why did National health programme focus on improving maternal health, improving child health and other endemic infectious tropical diseases? Have you ever scratched your head and asked to yourself why did International community, donors, policy makers invested on AIDS, African and Southeast region?

Because they had reasons in terms of global disease burden, mortality and disability rate and the conditions leading to it. Without those information, we wouldn’t have direction for public health planning, resource allocation and the impact of interventions. 

USAID is funding SUAAHARA project after corroborating the fact that under-nutrition is the underlying cause of many childhood death and maternal death.Our safe motherhood programme is focusing on access to attendance of skilled birth attendants and increasing institutional birth after the realization that home delivery is associated with high maternal and neonatal death. Information on the cause of mortality is crucial for future implications for reducing mortality.

VA has limitations

Nevertheless, Verbal autopsy is not free of limitations. They require recollection of events at the time of death, rely on understanding and reporting of signs and symptoms by interviewees, and may be influenced by interviewer skills. The result can be affected at any time of collection, Analysis &diagnosis, Reporting.

 

Download PDF for verbal autopsy questionnaire

Sources:

Frank Baiden, Ayaga Bawah, Sidu Biai, Fred Binka, Ties Boerma, Peter Byass, Daniel Chandramohan, Somnath Chatterji, Cyril Engmann, Dieltiens Greet, Robert Jakob, Kathleen Kahn, Osamu Kunii, Alan D Lopez, Christopher J L Murray, Bernard Nahlen, Chalapati Rao, Osman Sankoh, Philip W Setel, Kenji Shibuya, Nadia Soleman, Linda Wright, Gonghuan Yang. Bulletin of the World Health Organization. Setting international standards for verbal autopsy. http://www.who.int/bulletin/volumes/85/8/07-043745/en/

Hooman Khademi, Arash Etemadi, Farin Kamangar, Mehdi Nouraie, Ramin Shakeri, Behrooz Abaie, Akram Pourshams, Mohammad Bagheri, Afshin Hooshyar, Farhad Islami, Christian C. Abnet, Paul Pharoah, Paul Brennan, Paolo Boffetta, Sanford M. Dawsey, Reza Malekzadeh. Verbal Autopsy: Reliability and Validity Estimates for Causes of Death in the Golestan Cohort Study in Iran. Plos one. 2010. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0011183

Nadia Soleman, Daniel Chandramohan, & Kenji Shibuya. Verbal autopsy: current practices and challenges. Bulletin of the World Health Organization 2006;84:239-245.

 

Understanding of sentinel surveillance

Surveillance is a much talked topic in Public health course. This is an integral important part of in any health system for making it more responsive. However, we get muddled hearing number of terms that come with surveillance, especially when you are newbies to the surveillance system in health care. One of the frequently cited words among surveillance is sentinel surveillance. 

Surveillance is defined as “the ongoing, systematic collection, analysis, interpretation, and dissemination of data regarding a health-related event for use in public health action to reduce morbidity and mortality and to improve health”. Surveillance system keeps the track of any disease/conditions/risk factors and identifies any possibility of any outbreaks or epidemic early, helping to take steps for preparedness including early detection and treatment or management.

Surveillance can be categorised into Active Surveillance, Passive surveillance and Sentinel Surveillance, and disease specific surveillance. 

Sentinel Surveillance:

Case 1: BPKIHS,  is a sentinel surveillance center for Kalazar.

Why is it a Sentinel Surveillance center for Kalazar?

• It has commitment and willingness to remain as sentinel center for Kalazar
• It has all the laboratory diagnostic facilities to diagnose Kalazar
• It has skilled manpower doctors, nurses, paramedics required to manage/treat Kalazar cases
• It lies in Sunsari district, particular geographic area or population, and is near to the districts where the Kalazar is endemic 

What is it doing currently?

• It actively finds the cases in the community where kalazar is endemic: case detection
• It provides treatment for Kalazar and follow up
• (NOTE: BPKIHS could be sentinel surveillance centre for many other diseases)

With this example in background, we can clearly understand what sentinel surveillance is. It is a process of monitoring occurrence of particular conditions in a population or also a monitoring of disease occurrence in a specific cohort such as in a geographic area or population subgroup to estimate trends in a larger population. Always keep in mind that certain agencies are only selected as sentinel surveillance centre due to their capacities. 

Sentinel: a soldier or guard whose job is to stand and keep watch

Surveillance: close observation/monitoring

Like the way a soldier or guard is kept to monitor and check for anything for security in healthcare for certain diseases or risk factors, certain health centres are selected for surveillance of specified diseases and while doing so, that centre should meet all the prerequisites. 

Data collected in a well-designed sentinel system can be used to signal trends, identify outbreaks and monitor the burden of disease in a community, providing a rapid, economical alternative to other surveillance methods. Because sentinel surveillance is conducted only in selected locations, however, it may not be as effective for detecting rare diseases or diseases that occur outside the catchment areas of the sentinel sites.

1. Centers for Disease Control and Prevention. Updated guidelines for evaluating public health surveillance systems: recommendations from the guidelines working group. MMWR Morb Mortal Wkly Rep 2001;50(RR-13):2.

2. Sentinel surveillance. Immunization,vaccine and biologicals. http://www.who.int/immunization/monitoring_surveillance/burden/vpd/surveillance_type/sentinel/en/