It is because the sample size for the second survey was larger.
Survey research
A self-selected sample survey is when the surveyer choses who he or she wants to ask the survey question to. So, the people surveyed are not random at all. One problem with this is that the surveyer can easily manipulate results by asking a group of people specifically to get a certain result. For example, if I wanted to do a survey about abortion, and I wanted the results to be in favor of my liberal views, I might only survey people in a liberal coffee shop in Seattle. This is a self-selected survey.
The list is very long! In sampling error, I include all aspects of data collection. Samples (and not the full population) are taken in observational and experimental studies. The sample size may be a problem. In some cases, it may impossible to correct. If I am studying some rare occurrence, say hurricanes with winds over 280 mph or incidences of mad cow disease, the number of observations is fixed. Other times, there is inadequate time or budget to sample adequately. Sampling error can occur because of the way a sample is taken. This is very true of marketing surveys, which may be taken at time when they are more likely to survey one segment of the population. Or they are taken in one location that is not representative of the general population. A voluntary survey, or convenience survey may also be biased. The manner in which questions are posed, can introduce bias. Inadequate quality checking also contributes to sampling error. This is true whether the data collection is done by humans, or instruments, such as a testing laboratory. If a particular instrument is improperly calibrated, all measurements can be questionable. Finally, there are many means of purposely introducing bias into collected data in order to show "factual evidence" of preconceived ideas. The time frame or location where data is to be collected may be done to build in a particular bias. You will probably find more examples by searching the internet.
A convenience survey or a self-selection survey is most likely to be affected by bias
A population survey, better known as a census, entails the collection of each unit in the population. In sample survey information is collected from a subset of the population. The subset, or sample, needs to be selected carefully so that it is representative of the whole population and, if that requirement is met, statistics based on the sample are good estimators for the corresponding population parameters.
If the sample is not representative of the population, then the characteristics of the sample are not the characteristics of the population. Example: If I want to estimate the percentage of the population that are men, and my sample is the school's football team, my estimate would be that 100% of the population is comprised of men. What went wrong with my survey ? Simple. The football team is not a representative sample of the population, at least not as regards gender.
a stratified random sample
Sure it can. But in the survey business, the trick is to select your sample carefully so that they'll be equal, i.e. a sample that is accurately representative of the population.
double-blind selection
the participants are representative of the population they are interested in studying
A sample! And the large group is called the population.
The most important factor for accuracy in a survey is ensuring a representative sample of the population is surveyed. This helps to minimize bias and ensures that the results can be generalized to the larger population.
It's not.
The collection of information from a common group through interviews or the application of questionnaires to a representative sample of that group.
This type of research is called a survey study. Researchers use surveys to collect self-reported data from a sample of individuals to understand behaviors, attitudes, beliefs, or opinions in psychology. The goal is to generalize the findings from the sample to the broader population from which it was drawn.
a stratified random sample