The algorithm was virtually the same on the first four versions of watchOS, but, with the introduction of the Series 4 Apple Watch and Apple's introduction of its competing heartrate analysis app, Apple released watchOS5, which, among other things, "updated" the Watch's heartrate algorithm. That update did not improve the user experience for Apple Watch purchasers; instead, its purpose and effect was simply to prevent third parties from identifying irregular heartrate situations and, thus, from offering competing heartrate analysis apps.