The answer to whether or not standardized tests are fair to all students is, sometimes they are and sometimes they are not. In order for a test to be "fair", each question must measure one and only one thing. It also must measure the same thing for each student. There must be one and only one correct answer. The questions and answers must be culturally neutral; the wording should be free of rhetorical devices and there should be no clues to the correct answer within the question itself. Questions should be worded clearly using age appropriate language. A question should not be made more difficult through deliberately deceptive devices. Jargon or specialized terms should only be used when the candidates should have knowledge of these terms (e.g. physics terms are appropriate for a physics exam; not a math exam)
Those who construct the standardized test have the responsibility of seeing that their test meets this criteria. While standards are strict, many companies and governmental bodies cut corners to meet deadlines or budgetary requirements. A computational review can help to keep standardized tests fair. Questions that have impact should be thrown out; if a test can be shown to be biased, then it is not fair.
Student characteristics that might influence performance on a standardized test can range from the purely physical ones to emotional concerns or simply to mental aptitude. Physical abilities may impact performance in many ways. For instance, if a student is shown pictures and was asked to identify them, their vision would be a key factor. If there are oral instructions, hearing would be a factor. If a student had a headache, or they were hungry, they might be less able to concentrate. Concentration would also be affected by emotional problems, whether they were chronic as in some types of ADHD, or whether they were temporary such as grieving over a loss. Finally, all students are not equal in their mental abilities. With the emphasis today on testing the higher order thinking skills such as analysis or synthesis, some students will not be able to perform as well as others.
Standardized tests can be constructed to be very close to having a neutral bias, if care is taken. For instance, the testing environment can be altered for a student with ADHD. The student can be given a private room to take the exam. This will reduce the amount of distraction. Instructions can be read to them so that they will better understand what is required. However, there will always be individual exams that slip through the cracks and impact some students adversely. In addition, students bring their own biases into an exam situation. Prior experience and the human condition cannot be filtered out by the tester.