Lecture 2 - Sampling Flashcards
what is sampling? (definition)
predetermined procedure for selection, withdrawal, preservation, transportation and preparation of portions to be removed from a lot as samples
why do sampling?
- using a fraction of population is faster and cheaper
- analyzing a whole population is practically difficult
- gives info about whole population
increasing sample size increases…
- accuracy
- also time, cost, sampling methods, logistics of sample handling, analysis and data processing
appropriate sampling avoids…
- consumer risk
2. producer’s risk
what is consumer risk?
what is producer’s risk?
risk of accepting defective product
risk of rejecting an acceptable product
what are sampling and non-sampling errors?
sampling: arises when sample is not representative of population
non sampling: arises b/c of other reasons (ie environmental error from wrong strage)
what is variance?
total variance?
estimate of uncertainty
total variance: function of sum of variances associated w/ each step
- error occuring during sampling?
- during sample prep?
- during lab analysis?
- during data processing?
- during interpretation?
- sampling error
- erroneous sample prep
- manual or instrumental error
- erroneous data handling
- erroneous inference
factors that affect choice of sampling plans?
- purpose of inspection
- nature of population
- nature of product
- nature of test method
def of sample?
portion selected from large quantity of material. (general term)
- diff than lab sample
def of lab sample?
sample prepared for testing or analysis
def of population?
set of all objects in system being investigated
def of lot
quantity of bulk material of similar composition whose properties are under study
def of batch
quantity of food that is known or assumed to be produced under uniform conditions
batch number should always be noted when sampling foods
def of unit
Each of the discrete, identifiable units of food that are suitable for removal from the population as samples and that can be individually described, analyzed or combined (e.g. an apple, a bunch of bananas, a can of beans, a prepared dish).
def of homogeneity
degree to which a property or substance is randomly distributed throughout a material.
def of increment
An individual portion of material collected by a single operation of a sampling device.
- Increment can be tested individually or combined and tested as a unit.
def of increment
An individual portion of material collected by a single operation of a sampling device.
- Increment can be tested individually or combined and tested as a unit.
attribute vs variance sampling?
attribute:
- to decide on acceptability of a population based on whether the sample possess a certain characteristic or not
- 2 possible outcomes (conforming or nonconforming –> present or absent)
- sample size must be at least ten times smaller than population size
variance
- estimate quantitatively the amount of substance (ie proteint content or moisture content) or a characteristic (ie color) on a continuous scale
- esterimate obtained from sample is compared w/ an acceptable value
manual vs continuous sampling
manual:
- person doing sampling is trained
- picks samples randomly from sample lot
continuous:
- performed mechanically
- less prone to human bias than manual
3 criteria used to determine sample size?
- level of precision (or sampling error): defines desired level of precision when attributes are measured by using the sample
- level of confidence (or risk): 9% confidence level means 95 out of 100 samples have true population value within the range of precision specified
- degree of variability among population for attributes being measured: refers to distribution of attributes in population
sample size determination can be based on….
- precision analysis (confidence level)
2. power analysis (power of prediction)
Determine the sample size to test the total sugar in a lot of doughnuts with a level of confidence of 95 % and 5% accuracy (precision level). Preliminary test have estimated the average value to be 30 g of sugar per tray. The standard deviation was calculated to be 5g
n = ( (1.96 x 5)/(0.05 x 30) )^2
n=43
higher standard deviation means what?
a more heterogeneous sample
thus the sample size must increase
