Global Tuberculosis Control 2010 - Florida Department of Health

α=Ī Ī (1– Ī )
– 1
V
β=(1– Ī ) Ī (1– Ī )
– 1
V
ĪNV
V= 0.3
4
N
2
α= – x – x (1– – x )
V
– 1
β=(1– – x ) – x (1– – x )
V
– 1
x – V
(u – l) 2
V=
4
l = 1, u = 4
l = 0.01, u = 0.2
l = 0.2, u = 2
l = 0.01, u = 1
ƒ (x; α, β) =
x α–1 (1– x) β–1
u α–1 (1– u) β–1 du
1
0
Data were assessed using a three-step process. This
started with a systematic assessment of data quality,
including an assessment of the over-dispersion of count
data over time and across regions/districts (or similar
geographical areas). This was followed by exploration of
potential factors driving time-changes in case notifications
(such as improvements in diagnostic capacity and
the HIV epidemic), and then by assessment of the likely
number of TB cases that are not notified. To facilitate
the documentation of expert opinion, an “onion” framework
1 was used. In this framework, different “layers”
of the onion represent distinct populations of TB cases
that are not captured by national TB information systems
(for example, cases with no access to health care
and cases with access to private health-care services but
not reported to NTPs – see ).
These methods are documented in a workbook available
on the web site of the WHO Global Task Force on TB
Impact Measurement. 2
Distributions of CDRs for the three years for which
they were estimated were assumed to follow a Beta distribution
( ). Reasons for using Beta distributions
include the following:
They are continuous and defined on the interval (0,
1). Since the variance of CDR distributions tend to be
large as a result of high uncertainty, random draws
of numbers from a normal distribution would yield
numbers outside the interval (0, 1). The use of truncated
normal distributions may result in excess density
towards one of the bounds.
They are not necessarily symmetrical.
They are defined with two parameters that can be
estimated from available data using the method of
moments.
The two shape parameters necessary to define the
Beta distribution were computed using the method of
moments, as follows:
1
TB impact measurement: policy and recommendations for how to assess
the epidemiological burden of TB and the impact of TB control. Geneva,
World Health Organization, 2009 (Stop TB policy paper no 2; WHO/
HTM/TB/2009.416. The onion framework is described on pages
19–24). The policy paper is available on the Task Force’s website
http://www.who.int/tb/advisory_bodies/impact_measurement_
taskforce/en/index.html.
2
http://www.who.int/tb/advisory_bodies/impact_measurement_
taskforce/en/index.html.