What is downward closure property?

Every subset of a frequent itemset is also frequent. Also known as Apriori Property or Downward Closure Property, this rule essentially says that we don't need to find the count of an itemset, if all its subsets are not frequent. This is made possible because of the anti-monotone property of support measure - the support for an itemset never exceeds the support for its subsets. Stay tuned for this.