When, on a particular shopping trip, a consumer purchases an item which he previously had no intention of purchasing, this sale is called an "impulse purchase." The objects of impulse purchases are occasionally essential items (i.e. items that satisfy basic subsistence needs), but much more frequently are luxury or non-essential items. Researchers have determined that, at the end of a shopping trip, a consumer is much more excited if she has bought a luxury item on an impulse purchase, than if she had made no impulse purchases.
If the information above is true, and if the researchers' investigation was properly conducted, then which of the following must also be true?