CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...

2.8 STATISTICS
13-17-1 W G Warren
On testing whether a prescribed exclusion limit is attained
Introduction
The problem addressed is that of testing whether a prescribed exclusion
limit (fifth percentile) is attained in a well-defined population of plywood
panels. This population can be envisaged as, and may in actuality be, a
shipment. It is assumed that sampling of the shipment is effectively at random,
and that the sample size is a negligible fraction of the population
(shipment).
Several strategies will be considered:
(1) Fixed sample size, normality assumed
(2) Sequential sampling, normality assumed
(3) Fixed sample size, distribution free
(4) Sequential sampling, distribution free.
For (1) and (2) a study of the robustness against departures from normality
will also be undertaken.
Sequential sampling with normality assumed was considered by the writer
in a Supplemental Report (May, 1977). In this report the development of
the operating-characteristic curves is not correct; an approximation of the
unconditional distribution of the statistic x - ks was used instead of the
conditional distribution given that a decision had not been reached at a
previous stage. The implementation of the scheme is not affected but the
operating characteristics are in error, although it is difficult to ascertain the
magnitude of the error. Indeed, the curves obtained may well be reasonable
approximations.
A sequential strategy, based on the normality assumption, thus needs
redevelopment. Mathematically this is not an easy task. Since operational
sample sizes are urgently required, a fixed-sample size strategy will be
considered, firstly, with sequential methods to be discussed in a subsequent
note.
16-17-1 P Glos
Notes on sampling and strength prediction of stress graded structural
timber
Abstract
There are various factors that influence the quality of estimating characteristic
stress values. Among these is the definition of the specific population
about which inferences are to be made, decisions about the sampling
method and the sample size as well as the statistical definition of the characteristic
stress value and the choice of the statistical model used for its
computation.
For many of the decisions mentioned above the statistical theory does
not offer satisfactory decision rules. Hence in some cases the engineer
must decide intuitively. Regarding the harmonization of codes and standards
it would be helpful if such decisions were standardized internationally.
As a contribution to the discussion of this topic this paper summarizes
the potential errors that may arise when estimating the 5 percent exclusion
limit and tries to assess their magnitude as a function of the sample size
and of the underlying statistical assumptions.
16-17-2 B Norén
Sampling to predict by testing the capacity of joints, components and
structures
Background
During the last ten years the working group CIB-W18 has concentrated on
an European model code for timber structures: CIB Structural Timber Design
Code. Design methods are dominating in this code in terms of formulas
for calculating deformation and strength of structures and structural
components. Introductionary are also presented strength classes for structural
timber and glued laminated timber with the correspondent profiles of
characteristic strength at different kind of stressing.
Additionally, CIB-W18 has dealt with methods of verifying characteristic
values of stiffness and strength of material, joints, components and
structures of wood. This work had been carried out in subgroups set up by
CIB and RILEM rather informally. The results are passed as proposals to
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.72