My neighbor has a fruit tree right on the fence line. At this time of year, it typically drops a little fruit in my yard. But this year was different. Fruit has been dropping for nearly two months. And it is usually half eaten. So, I blamed the squirrels. Until one day when one of the branches was jerking around wildly. I watched for a minute as a crow became visible, hanging upside down, pecking and jerking the fruit until it was partially eaten and fell to the ground. I had it wrong!
The thing about assumptions is, they can turn out to be wrong. Even when using the best information available. And in business, it is important to clearly state the assumptions that underlie plans. Whether it is adding new capacity, specific market conditions, adoption of a new product, etc., the assumptions need to be laid out. That way, people can focus on making the assumptions happen to get the outcomes they expected.