Lecture 2 - Sampling Flashcards
define sampling
predetermined procedure for the selection, withdrawal, preservation, transportation, and preparation of the portions to be removed from a lot as samples
why do we sample?
- save $$$
- difficult logistics of large populations
the set of all objects in the system being investigated
population
a portion selected from a large quantity of material
or a unit taken from the total amount of food
sample
a sample prepared for testing or analysis
laboratory sample
a quantity of bulk material of similar composition whose properties are under study
lot
quantity of food that is known or assumed to be produced under uniform conditions
batch
each discrete, identifiable portion of food that are suitable for removal from a population as samples that can be individually describes, analyzed or combined
unit
the degree to which a property or substance is randomly distributed throughout a propulation
homogeneity
an individual portion of material collected by a single operation of a sampling device
increment
accurate sampling is important to avoid what kind of risks?
consumer risk, producer risk
what is consumer risk?
accepting defective product
what is producer risk?
rejecting an acceptable product
_____ error arises when a sample is not representative of the population
sampling
_____ error arises because of some other reasons
non-sampling
______ is an estimate of the uncertainty
variance
______ ______ is a function of the sum of variances associated with each step
total variance
what are the probability sampling plans?
- simple random sampling
- stratified sampling
- cluster sampling
- composite sampling
- systematic sampling
what are non-probability sampling plans?
- judgment sampling
- convenience sampling
- restricted sampling
- quota sampling
every unit in a population having a known and equal chance of being selective is representative of what category of sampling plan?
probability sampling plan
what motivates the use of a probability sampling plan? (4)
- provides statistically sound basis for obtaining representative samples
- eliminate human bias
- scientific, operationally convenient and simple in theory
- best methods to achieve a representative sample
describe simple random sampling
- population N must be known
- random selection process
~ simple, high prob of being representative
~ IDing all units of the pop can be difficult
describe stratified sampling
- divides pop into mutually exclusive subgroups then uses simple random sampling to choose units from each stratum
~ more homogenous data, administrative convenience, lower cost, smaller variances for sample sizes
~ potential classification error
~ prior knowledge of composition and distribution of population; tedious
describe cluster sampling
- pop divided into clusters designed to be as similar as possible; randomly selected clusters sampled
~ less costly than simple/stratified
~ higher sampling error
describe systematic sampling
- first unit selected at random from the first k population units and then units are taken every nth unit
~ highly representative, very informational
~ not as random
describe composite sampling
- samples drawn from different units in a pop are pooled for analysis
the technique where the odds of any unit being selected for a sample cannot be calculated since the selection is based on nonrandom procedures refers to _____ sampling plans
non-probability
when would non-probability sampling be appropriate?
checking for adulteration
describe judgment sampling
dependent on subjective judgment of the person who is drawing the samples
describe convenience sampling
- selecting only accessible part of pop
- aka chunk or grab sampling
describe restricted sampling
used when the entire pop is not accessible
describe quota sampling
- pop divided into groups; samples are chosen based on experience and judgment
(sampling from groups representing various categories)
what 4 factors affect the choice of sampling plan?
- purpose of inspection
- nature of population
- nature of product
- nature of test method
what’s attribute sampling?
- based on whether or not sample possess a certain characteristic (discrete)
what’s variance sampling?
- estimate quantitatively the amount of a substance or a characteristic (continuous)
what type of sampling is this: trained person randomly picks sample/location from sample lot/liquid container as specified by the approved standard method
manual sampling
what type of sampling is this: performed mechanically
continuous sampling
what 3 criteria determine sample size?
- level of precision
- level of confidence (risk)
- degree of variability among the population for the attributes beingmeasured
write the sample size formula for an unknown population
okay
write the sample size formula for a known population
aight
