The use of bottom-up and top-down processing depends on the context. In some cases, individual words require attention (bottom-up), while in other cases the entire context requires attention (top-down). Bottom-up uses chunks and breaks down individual words to understand the meaning of the words before combining the entire chunk currently playing. This will sometimes lead to comprehension problems as contextual awareness is used. The top-down approach is more common in classrooms but is not necessarily the most effective approach for reading comprehension. The reader uses the overall theme or goal to gain insight. This is sometimes not effective because individual learner variables (race, culture, native language, etc.) cannot place the text in context without individual knowledge of the words. These two approaches combined will be effective in a reading classroom, but may not be effective in a regular educational setting. These approaches place limits on teachers attempting to meet curriculum standards and guidelines. The time factor required to implement these two approaches would be demanding for a regular classroom.