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A walk in the Park

This blog is dedicated to everyone who has struggled with Community Medicine. Through my posts I hope to simplify and demystify community medicine. The emphasis will be on clarifying concepts rather than providing ready-made answers to exam questions.

Feedback is crucial for the success of this endeavour, so you are encouraged to comment and criticize if you cannot understand something.

If you want a topic to be discussed sooner rather than later, please let me know via

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Twitter: @DocRoopesh

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A single example may not be able to explain 100% of a given topic, so multiple examples may be provided to explain different parts of a single concept.

If something doesn’t seem right:

a. Write to me about it (at communitymedicine4asses@yahoo.com), and

b. Cross check with another source (textbook, expert, etc.)

I hope that my exertions will make your experience with community medicine seem like a “Walk in the Park”

Note 1. Those who wish to contact me on facebook are requested to kindly send a personal message introducing themselves along with the request. This will help save time and effort of all concerned. Please do not expect me to visit your page to try and identify you/ your areas of work/ interest, etc. It is common courtesy to introduce oneself to another when interacting for the first time. I am merely requesting that the same civil courtesy be extended here, too. Henceforth, I may not accept any friend requests/ requests to join the group on facebook unless accompanied by a note of introduction (except when I already know the sender).  

Note 2. Please understand that this blog (and the corresponding facebook page/ group) is maintained in my spare time. I have a full time job, and am available to pursue these activities only after regular working hours (after 5 pm Indian Standard Time). However urgently you may wish to receive a response from me, I will be able to respond only upon returning home from work (I am offline the rest of the time).

Note 3. Please mind your language when interacting with me/ in the group linked to this blog. Rude/ offensive language will result in expulsion from both my friends list and the said group.

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Choosing the appropriate statistical test: Part 2a: Continuous, Normally Distributed Outcome Variable

Disclaimer: This article is primarily intended for an online group of post graduate students in Community Medicine that I am involved with. The group was created to provide supplemental instruction to members on topics of common interest. Instruction is in bite-sized portions, since all members are busy PG students. Conceptual understanding is emphasized. Membership to that (Whatsapp) group is through invitation only. However, others interested in participating in the discussions and related activities in Google classroom may indicate the same by sending me a message on Facebook.

This article will focus on situations wherein the outcome variable is continuous, and normally distributed.

When I say a variable is normally distributed, I mean that the data follow a normal (Gaussian) distribution- the mean, median and mode coincide (are the same/ similar), has a bell-shaped curve (when visualized using a histogram), etc.

Why is it important to assess if a continuous variable is normally distributed or not? Simply because it influences the choice of statistical test of significance available for hypothesis testing. When data are normally distributed, one can employ ‘parametric’ tests of significance. Typically, when using such tests of significance, the mean assumes importance (the test(s) see(s) if there is a difference between means). Generally, when the sample size exceeds 30, the values follow a normal distribution. However, one must verify of this is true before applying parametric tests of significance.

Let us consider various scenarios when the outcome variable is continuous, and normally distributed.

Scenario 1: One wants to know if there is a difference in mean values between two groups that are unrelated (The outcome/ response variable is continuous and normally distributed; predictor variable is categorical, and has two levels.).

Example: One wants to know if the mean income varies by sex. Here, the response variable is income, and is normally distributed. The predictor variable is sex, and has two levels- male and female. The income of males is not related to that of females and vice-versa (the two are ‘independent’ of each other).

Appropriate statistical test: Independent-samples t Test

Scenario 2: One wants to know if there is a difference in mean values before and after some intervention (The outcome/ response variable is continuous and normally distributed).

Example: One wants to know if the mean weight varies before and after exercise. Here, the response variable is body weight after exercise, and is normally distributed. Unlike the previous scenario, the predictor variable is also body weight, but before exercise. Here, the post-exercise weight is ‘dependent’ on the pre-exercise weight.

Appropriate statistical test: Paired-samples t Test

Scenario 3: One wants to know if there is a difference in mean values between more than two groups that are unrelated (The outcome/ response variable is continuous and normally distributed; predictor variable is categorical, and has more than two levels.).

Example: One wants to know if the mean weight varies by social class (there are more than two social classes- Low, Middle, High). The social classes are mutually exclusive- one cannot belong to more than one social class at any point in time.

Appropriate statistical test: Analysis of Variance (ANOVA)

Note: When the predictor categorical variable has more than two levels, a direct comparison of means is not feasible, therefore, regression methods are used to determine if there is a statistically significant difference between any two levels of the categorical variable. The results of ANOVA only indicate that there is a significant between two levels, but does not identify which two levels are responsible for the same. In order to determine the specific levels causing statistical significance, one needs to apply one of many post-hoc tests. This will pinpoint the two levels having significant difference in values.

Useful Link:

Link to previous article in the series:

https://communitymedicine4asses.com/2020/02/10/choosing-the-appropriate-statistical-test-part-1general-considerations/