When new data is discovered that does not fit the current hypothesis, the hypothesis must be modified to explain the new data. A wonderful modern example of this is shown in the sinking of the Titanic. Before the shipwreck was discovered, it was widely believed that the iceberg created a huge hole in the ship. But upon analysis of the wreckage, it was found that the hole that destroyed the Titanic covered about as much surface area as a common front door to a house. This new data forced historians to change their idea about the collison that sank the Titanic.
Historians are prone, like most people are, to opinions based on their own experiences.
Historians typically choose to study parts of history that interest them. So typically, historians have a preconception, however small, about the period or culture they want to study. The key is not to let your ideas of a culture or a period in history interfere with our analysis of the data that was collected about them. Common people in history are not known about for some very simple reasons. First common people are not as intersting so no record was kept of their accomplishments, that is why they are called common.