玩；t为预计孩子定期玩玩具的时间（小时为单位）；l代表孩子被该玩具吸引的总共时长（以月为单位）；c代表玩具价格。
a psychologist has come up with a formula for parents struggling to find the perfect toy for their children this christmas.
dr cliff arnall, who has previously come up with calculations for the happiest and most depressing day of the year, reckons that mathematics can be used to work out the "play value" of potential purchases.
but parents hoping for a simple formula to help them make wise decisions in the toyshop had better not be in a hurry and might have to bring a calculator with them.
the result is also likely to be different for different children, he admits.
parents first have to assess the toy against six criteria: its usefulness when playing alone (pi); playing with others (po); whether it fosters creativity (cr); its value promoting social interaction (s); its general utility (u) and finally whether it can be handed down to siblings (h).