Lecture 2- Sampling Flashcards
What is sampling?
Predetermined procedure for the selection,withdrawal, preservation transportaion and preparation of the portions to be removed from a lot as samples
Why do we do sampling?
Easier, More cost effective, quicker
Get imformation about LARGE populations
Population
Set of all objects in the system being investigated
Sample
Portion selected froma large quantity of material –> term used for a unit taken from teh total amount of food
Laboratory sample
Sample prepared for testing or analysis
Lot
Quantity of bulk material of SIMILAR composition whose properties are under study
Batch
Quantity of food that is known, or assumed to be produced under UNIFORM conditions
- # should be notes
Unit
Each of the discrete, identifiable portion of food that are suitable for removal from a population as samples that can be individually described, analyzed or combined
Homogenity
The degree to which a property or substance is randomly distrubuted throughout a population
Increment
Individual portion of material collected by a single operation of a sampling device
What are the positive and negative aspects of increasing sample size?
+ Increase accuracy
- Increase time and cost of sampling, handling, analysis and data processing
Why is it important to sample properly?
To avoid:
Consumer risk –> accepting a defective product
Producer risk –> rejecting an acceptable product
What is an example of sampling error?
Sample is not representative of the population
What is an example of a non-sampling error?
Transferring data from the questionnaire
What is variance?
An estimate of the uncertainty
What is total variance?
Sum of variance at each step
What is an example of error at... at the 1. Sample collection 2. Sample preparation 3. Laboratory analysis 4. Data processing 5. Interpretation
- Sampling error
- Erroneoud sample preparation
- Manual or intrument error
- Erroneous data handling
- Erroneous inference
What ar ethe two types of sampling plans?
Probability and non-probability plans
What is the basis of probaility sampling plans?
Every unit of a population has a known and equal chance of being selected
- eliminate human bias
- statistically sound basis
- best for a representative sample
What is the simple random sampling?
Each element of the population has an equal chance of being included in the sample–> uses a random selection process
**Population needs to be known
What are the advantages of simple random sampling?
Easy to conduct
High probability of acheiving a representative sample
Meets assumptions of statistical procedures
What are the limitations of simple random sampling?
Indentification of all units of the population can be difficult
What is stratified sampling?
Dividing the population into mutally exclusive subgroups(strata) and then using simple random sampling to choose units from each stratum
What is an example of stratified sampling?
Egg shell packing station the boxes of eggs are placed on pallets according to their grade size, the population is naturally divided into strata
What are the advantages of stratified sampling?
- Data is more homogenous within each stratum
- Cost seems to be lower due to admin
- Small variances for the sample
What are the disadvantages of stratification?
Classification error
Prior knowledge of composittion and distribution of population
Tedious and time consuming (especially with larger samples
What is cluster sampling?
The population is divided into cluster that are designed to be similar as possible to one another
Only some randomly selected clusters are sampled
** Stratified are taken from every single subgroup
What is the advantage of cluster sampling?
Less costly than simple or stratified
What is a disadvantage of cluster sampling?
Higher sampling error (lack of representing
What is systematic sampling?
The first unit is selected at random start from the first k population units and then units are taken every nth unit N population Sample size n K=N/n Select a number j between 1 and k j, j+k, j+2k, j+3k, j+4K....
What are the advantages of sstematic sampling?
Highly representative, w/o random number generator
Easier to preform
Less prone to error
Provide information per unit cost
What are some disadvantages of systematic sampling?
Not as random as simple random sampling
What is composite sampling?
Collection of numerous individual discrete sample at regular intervals (over period of time)
Used obtained samples from bagged products
Small portions are taken from different locations, bags, containers
Composite sample may be physical mix of individual samples units
What are the 5 factors of sampling plans?
- purpose of inspection
- Nature of population
- Nature of product
- Nature of test method
What are the non-probabilty sampling plans?
Judgement
Convinience
Restricted
Quote
What is judgement sampling?
Dependent on the subjective judgement of hte person who is drawing the samples
What is convinence sampling?
Selecting only accessible part of the population –> chunk sampling or grab sampling
Restricted Sampling
sampling (non-probability)
The population is divided into groups, and samples are chosen based on experience and judgement
What is attribute sampling?
To decide on the acceptability of a population based on whether the sample possesses a certain characteristics or not
Either present or absent
** Sample should be ten time smaller than the population size
What is variance sampling?
Performed to estimate quantitatively the amount of a substance or a characteristic on a continuous scale - in a range
** obtained when compared with acceptable value
What is manula sampling?
Person doing the sampling should be trained, pick up samples randomly
Continuous sampling
Performed mechanically
Less prone to human bias
What is the sample size of attribute sampling?
10x smaller than the population
Which sampling needs more samples?
Attribute over variance
Why does attribute sampling need a larger sample?
In order to make a right judgement
