Types of surveys The population is the total number of items under consideration. For example, the population of Australia is approximately 22,000,000 or the student population of this school is approximately 420. A census is a survey that includes every member of a population. If a population is too large or too difficult to survey (eg large distances between people), a sample of the population may be surveyed.
Census or Sample? • Eg 1: Give an example of when the students in this school would be: • a census • a sample
Census or Sample? • Eg 2: Give an example of when the population of Bundanoon would be: • a census • a sample
Types of samples • If a sample is to be truly representative of the population, it needs to be: • Statistically large enough to draw conclusions from • Eg surveying 50 students at this school is a “large enough” sample • But surveying 50 Australians about a national issue is not enough. • Selected without bias. • If a students were being surveyed about a new item in the canteen, students should be chosen from all years, not just year12.
Types of samples Some types of samples include: Random sample Here each member of the population is equally likely to be chosen, so the sample has attributes (properties) of the whole population. An example is picking a name out of a hat. Systematic sample Here one member of the population is chosen then other members at a regular interval. An example is picking every 20th light globe off the assembly line. Stratified sample Here the population is broken into strata or layers, eg years at school. Now a random sample is chosen from each stratum. An example would be if 45% of the population are female then 45% of the sample should also be female.
Privacy and ethics The collection and recording of data can intrude on the privacy of others, especially when names, addresses and phone numbers are recorded. Steps need to be taken to ensure that the information is kept securely and confidentially. Access to the data should be limited to those with a genuine need for it. More generalised data or data without names and addresses etc may be more appropriate for some uses. Your private data may be sold by the data collector to others, eg people access electoral rolls to conduct other surveys. Some incentive or rewards schemes (eg flybuys) may provide data they collect to third parties. Just be careful who you give your details to!!
Example 1 - Stratefied The Bundanoon Bridge club is going to survey 50 of its members about their preferences for morning tea. How many members should it ask from each group? How many members? 200 Begin-F Adv-F Int-F Begin-M Adv-F Int-F Check: 12 + 7 + 5 + 17 + 7 + 2 = 50
Example 2 - Systematic The Bundanoon Battery Coop wants to test 25 batteries out of a batch of 490 that they have produced. How could they conduct a systematic sample? Batteries are from 1 to 490. 490 25 = 19.6 So start at any number (less than 19) and then select every 19th battery until you have a sample of 25 batteries
Today’s work Exercise 5D pg 160 #2a, c, e, g Exercise 5J pg 170 #4ab, 6ab, 9ab Exercise 5K pg 173 #1a, 2a, 3a, 6ab Exercise 5L pg 175 #1 to 10