2 Preliminaries
where θ ⊂
θ
is unknown. The distribution may depend also on design fea-
tures of the study that generated the data. We typically simplify the notation to
f
Y
(y; θ), although the explanatory variables z are frequently essential in specific
applications.
To choose the model appropriately is crucial to fruitful application.
We follow the very convenient, although deplorable, practice of using the term
density both for continuous random variables and for the probability function
of discrete random variables. The deplorability comes from the functions being
dimensionally different, probabilities per unit of measurement in continuous
problems and pure numbers in discrete problems. In line with this convention
in what follows integrals are to be interpreted as sums where necessary. Thus
we write
E(Y ) = E(Y ; θ) =
yf
Y
(y; θ)dy (1.2)
for the expectation of Y, showing the dependence on θ only when relevant. The
integral is interpreted as a sum over the points of support in a purely discrete case.
Next, for each aspect of the research question we partition θ as (ψ , λ), where ψ
is called the parameter of interest and λ is included to complete the specification
and commonly called a nuisance parameter. Usually, but not necessarily, ψ and
λ are variation independent in that
θ
is the Cartesian product
ψ
×
λ
. That
is, any value of ψ may occur in connection with any value of λ. The choice of
ψ is a subject-matter question. In many applications it is best to arrange that ψ
is a scalar parameter, i.e., to break the research question of interest into simple
components corresponding to strongly focused and incisive research questions,
but this is not necessary for the theoretical discussion.
It is often helpful to distinguish between the primary features of a model
and the secondary features. If the former are changed the research questions of
interest have either been changed or at least formulated in an importantly differ-
ent way, whereas if the secondary features are changed the research questions
are essentially unaltered. This does not mean that the secondary features are
unimportant but rather that their influence is typically on the method of estima-
tion to be used and on the assessment of precision, whereas misformulation of
the primary features leads to the wrong question being addressed.
We concentrate on problems where
θ
is a subset of R
d
, i.e., d-dimensional
real space. These are so-called fully parametric problems. Other possibilities
are to have semiparametric problems or fully nonparametric problems. These
typically involve fewer assumptions of structure and distributional form but
usually contain strong assumptions about independencies. To an appreciable