Survivorship bias, survival bias or immortal time bias is the logical error of concentrating on the people or things that made it past some selection process and overlooking those that did not, typically because of their lack of visibility. This can lead to incorrect conclusions.
Survivorship bias is a form of selection bias that can lead to overly optimistic beliefs because multiple failures are ignored, such as when companies that no longer exist are excluded from analyses of financial performance. It can also lead to the false belief that the successes in a group have some special property, rather than just coincidence (correlation "proves" causality). For example, if three of the five students with the best college grades went to the same high school, that can lead one to believe that the high school must offer an excellent education when, in fact, it may be just a much larger school instead. This can be better understood by looking at the grades of all the other students from that high school, not just the ones who made the top-five selection process.
Another kind of survivorship bias would involve thinking that an incident was not all that dangerous because everyone communicated with afterwards survived. Even if one knew that some people are dead, they would not have their voice to add to the conversation, leading to bias in the conversation.