No selection bias is taking specific data in order to fulfill a bias. So the bias would be the Biblical Flood is historic. One supporting this idea takes "flood stories" from other cultures as evidence. They use only selective data to support the case. Selective data would be the event, a flood. So due to there being other floods one puts forward these stories correlate with the Biblical one. However these stories also contain other details such as in Chinese flood stories in which the flood was ended by man not a god and never become global. One omits this data in order to meet their bias. The details do not matter, the narratives do not matter, the outcome of the story does not matter. The only thing that matters is that a flood happened. So while there are hundreds of flood stories most do not share any similarity to the Biblical one once the details are examined. As with the Epic of Gilgamesh the narratives details show a pattern of writing which is also in the Biblical story. The details show the story was lifted from Babylon/Sumeria and placed into the Biblical one due to the details rather than focusing on the ambiguous term of a flood. Minor details are changed such as the people, the scale of the event but the narrative remains the same in form. The epic matches a logical scenario as it limits the scale of the event. It does not put forward a complete destruction of humanity down to 8 people. This makes the story more plausible since it does not make absurd leaps in the narrative which has drastic implications in reality, such as there being only 8 people alive.
A non-flood example is "Ancient Aliens" theory. The presupposition is that humans were not capable of building various structures around the world; temples, pyramids, etc. To give this idea more credibility those that support the idea look for any text mentioning spirits, gods, angels, etc, from the sky. These texts are retrofitted into the theory thus these entities become aliens. They ignore the details and focus on entities from the sky coming to down to Earth. Every other detail is dismissed. Thus aliens did it. Supporters only select data which supports their view. Every piece of data that does not is omitted on purpose. Thus selection bias.