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The principles of Bayesian analysis date back to the work of Thomas Bayes, who was a Presbyterian
minister in Tunbridge Wells and Pierre Laplace, a French mathematician, astronomer, and physicist in
the 18th century. Bayesian analysis started as a simple intuitive rule, named after Bayes, for updating
beliefs on account of some evidence. For the next 200 years, however, Bayes’s rule was an
obscure idea. Along with the rapid development of the standard or frequentist statistics in 20th century,
Bayesian methodology was also developing, although with less attention and at a slower pace. One
of the obstacles for the progress of Bayesian ideas has been the lasting opinion among mainstream
statisticians of it being subjective. Another more-tangible problem for adopting Bayesian models in
practice has been the lack of adequate computational resources. Nowadays, Bayesian statistics is
widely accepted by researchers and practitioners as a valuable and feasible alternative.
Bayesian analysis proliferates in diverse areas including industry and government, but its application
in sciences and engineering is particularly visible. Bayesian statistical inference is used in econometrics
(Poirier [1995]; Chernozhukov and Hong [2003]; Kim, Shephard, and Chib [1998], Zellner [1997]);
education (Johnson 1997); epidemiology (Greenland 1998); engineering (Godsill and Rayner 1998);
genetics (Iversen, Parmigiani, and Berry 1999); social sciences (Pollard 1986); hydrology (Parent
et al. 1998); quality management (Rios Insua 1990); atmospheric sciences (Berliner et al. 1999); and
law (DeGroot, Fienberg, and Kadane 1986), to name a few.
The posterior density (shown in red) is more peaked and shifted to the left compared with the prior
distribution (shown in blue). The posterior distribution combined the prior information about with
intro — Introduction to Bayesian analysis 3
the information from the data, from which y = 0 provided evidence for a low value of and shifted
the prior density to the left to form the posterior density. Based on this posterior distribution, the
posterior mean estimate of is 2=(2 + 40) = 0.048 and the posterior probability that, for example,
< 0.10 is about 93%.
If we compute a standard frequentist estimate of a population proportion as a fraction of the
infected subjects in the sample, y = y=n, we will obtain 0 with the corresponding 95% confidence
interval (y �� 1.96
p
y (1 �� y)=n; y + 1.96
p
y (1 �� y)=n) reducing to 0 as well. It may be difficult
to convince a health policy maker that the prevalence of the disease in that city is indeed 0, given
the small sample size and the prior information available from comparable cities about a nonzero
prevalence of this disease.
Bayesian and frequentist approaches have very different philosophies about what is considered fixed
and, therefore, have very different interpretations of the results. The Bayesian approach assumes that
the observed data sample is fixed and that model parameters are random. The posterior distribution
of parameters is estimated based on the observed data and the prior distribution of parameters and is
used for inference. The frequentist approach assumes that the observed data are a repeatable random
sample and that parameters are unknown but fixed and constant across the repeated samples. The
inference is based on the sampling distribution of the data or of the data characteristics (statistics). In
other words, Bayesian analysis answers questions based on the distribution of parameters conditional
on the observed sample, whereas frequentist analysis answers questions based on the distribution of
statistics obtained from repeated hypothetical samples, which would be generated by the same process
that produced the observed sample given that parameters are unknown but fixed. Frequentist analysis
consequently requires that the process that generated the observed data is repeatable. This assumption
may not always be feasible. For example, in meta-analysis, where the observed sample represents the
collected studies of interest, one may argue that the collection of studies is a one-time experiment.
Multiple datasets in memory in Stata 16
You can now load multiple datasets into memory. You type
. use people
and people.dta is loaded into memory. Next, you type
. frame create counties
. frame counties: use counties
and you have two datasets in memory. people.dta is in the frame named default, and counties.dta is in the frame named counties. Your current frame is still default. Most Stata commands use the data in the current frame. For example, if you typed
. list
then people.dta will be listed. If you typed
. frame counties: list
then counties.dta will be listed. Or you could make counties the current frame by typing
. frame change counties
and list will now list the counties data.
Navigating frames is easy and so is linking them. Imagine that both datasets have a variable named countycode that identifies counties in the same way. Type
. frlink m:1 countycode, frame(counties)
and each person in the default frame is linked to a county in the counties frame. This means you can now use the frget command to copy variables from the counties frame to the current frame. Or you can use the frval() function to directly access the values of variables in the counties frame. For instance, if we have each individual’s income in the default frame and median county income in the counties frame, we can generate a new variable containing relative income by typing
. generate rel_income = income / frval(counties, median_income)
This is the beginning. While this example uses only two frames, you can have up to 100 frames in memory at once, and you can have many links among those frames.
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