According to the Cornell National Social Survey 42% of survey respondents withheld information when they had something to lose by being candid. Some of the concerns employees can have when deciding whether to be honest giving feedback can be
Threat to job security
Despite promises to the contrary many employees can feel like providing negative feedback to their employer can be construed as the employee exhibiting a poisonous attitude, which can and does lead to termination. For most people the risk of negative consequences up to and including termination is simply not worth it.
Risk of being misunderstood
Surveys are not a perfect means of communication and often what was meant as constructive feedback can come across as vindictive or malicious. For this reason many people will play it safe with their feedback if they know they can be linked back to their answers later. Better to say nothing than to say something that can be misinterpreted to this extreme.
Aversion to standing out
All humans have an ingrained need to belong and remain part of a group, whether that is in social situations, relationships, or indeed in their employment. For many people giving negative or otherwise out of the ordinary feedback to their employer can trigger the fear of exclusion. We all want to be "team players" and will often mute our own feelings in order to maintain the status quo of the group. Anonymity can provide the shield people need to be more candid.
Factors than can compromise anonymity
The larger the group of employees surveyed (the sample size), the harder it will be to identify one answer. For example, if you have a group of five employees one of whom is on a performance improvement plan, or is otherwise currently undergoing a period of friction with the organisation, then if one of the feedback surveys comes back as negative it can be relatively easy to identify the individual. If you had 500 employees then identifying the individuals responsible for a handful of negative responses becomes significantly more difficult.
Do you have one employee who gets in before everyone else and checks their emails in the office on their own? If your first survey result is submitted at the time this employee arrives at work then this can act as an identifying factory. With Acorde surveys we do not expose the time a survey was completed for this very reason. Knowing when employees submit their survey results is not particularly insightful information and the risk of compromising anonymity can be significant.
Any question that allows an employee to write an answer in their own words is very difficult to keep anonymous. If employees are given free reign to submit feedback in their own words then nothing can be done to guarantee the information they submit does not identify them. From mentioning their co-workers or their manager by name to identifying a specific project or client the employee has worked in can be enough to easily identify the individual. Even language and writing style can be used in some cases. As a rule, it is generally recommended to treat any feedback that allows employees to write text freely as confidential and potentially identifying information about that individual.