As a contractor, I only accept per hour since this is the best result for both parties. A fixed sum for a task requires a fixed and extremely detailed spec up f

As a contractor, I only accept per hour since this is the best result for both parties. A fixed sum for a task requires a fixed and extremely detailed spec up front which leads to worse software, since you always learn more in the process. And even then there may be ambiguities and misunderstanding which means the contractor might have to use more time than expected to satisfy the customer.