Accurate sample size calculation is important for finding reliability.
To do power analysis to estimate your sample size, you have to write your hypothesis, and based on that you decide what statistical test you will use. It should be one of the inferential statistics. so you need to determine the following: alpha {standard to be .05}, power [standard to be .80], effect size {small, moderate, or large, each test has its own value, you can find these values in the net}. Then download free programs to calculate the sample size such as G. power.
Try this Mohammed
https://www.researchgate.net/publication/262564897_Easy-to-use_Sampling_Formulae
Trend design. It is a design where a new sample is drawn at each measurement period to keep up with changes that may have occurred in the general population.
GS1 GS2 GSn where GS= sample from the general population
T1 O11
T2 O22
Tn O3n
Cohort design. It is a variation in trend design in which specific subpopulations, or cohorts, are examined as they change overtime. A cohort is a group of people that have a certain characteristic in common, such as being born in the 1950, graduating from college in 2005, or marrying between 1990 and 2000. The same population is involved throughout the research but a fresh sample is selected every time data are gathered.
CS1 CS2 CSn where CS= sample from the cohort population
T1 O11
T2 O22
Tn O3n
Data Easy-to-use Sampling Formulae
This might be a good first start: https://onlinecourses.science.psu.edu/stat507/node/54
http://www.openepi.com/SampleSize/SSCohort.htm
From my experience I can recommend free software for power sample calculation developed at University of Dusseldorf - G*Power (http://www.gpower.hhu.de/en.html). It has comprehensive tutorial at their web pages and very detailed output compared to other software solutions.
Mohamed,
There are several online calculators that are free to use, just be aware of their assumptions and limitations.
Keep in mind that there is a lot of "art" to calculation power/sample size for a given study. You make a lot of assumptions that are made, much of which is based on expert opinion. Issues like effect size, confounding, and proportion of the population with the exposure can be challenging. Good luck.
http://epitools.ausvet.com.au/content.php?page=SampleSize
http://clincalc.com/Stats/SampleSize.aspx
To do power analysis to estimate your sample size, you have to write your hypothesis, and based on that you decide what statistical test you will use. It should be one of the inferential statistics. so you need to determine the following: alpha {standard to be .05}, power [standard to be .80], effect size {small, moderate, or large, each test has its own value, you can find these values in the net}. Then download free programs to calculate the sample size such as G. power.
use this formula for a cohort study
https://www.google.com.ng/search?q=Sample+Size+for+a+Cross-Sectional%2C+Cohort%2C+or+Clinical+Trial+Studies&oq=Sample+Size+for+a+Cross-Sectional%2C+Cohort%2C+or+Clinical+Trial+Studies&aqs=chrome..69i57j0.1174j0j1&sourceid=chrome&ie=UTF-8
Many tend to confuse cohort studies with case control studies....For example, this author https://www.slideshare.net/drtamil/5-calculate-samplesizeforcasecontrol and https://www.slideshare.net/drtamil/6-calculate-samplesize-for-cohort-studies uses the same formula for case control and cohort.
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