我正在使用EdX开发Analytics Edge课程并遇到了这个问题。我们有一个数据集，我们正在进行子集化。在子集上运行Str按预期工作，但是对同一子集执行摘要会引发错误。有人可以解释原因吗？
> str(WHO_Europe)
'data.frame': 53 obs. of 13 variables:
$ Country : Factor w/ 194 levels "Afghanistan",..: 2 4 8 10 11 16 17 22 26 42 ...
$ Region : Factor w/ 6 levels "Africa","Americas",..: 4 4 4 4 4 4 4 4 4 4 ...
$ Population : int 3162 78 2969 8464 9309 9405 11060 3834 7278 4307 ...
$ Under15 : num 21.3 15.2 20.3 14.5 22.2 ...
$ Over60 : num 14.93 22.86 14.06 23.52 8.24 ...
$ FertilityRate : num 1.75 NA 1.74 1.44 1.96 1.47 1.85 1.26 1.51 1.48 ...
$ LifeExpectancy : int 74 82 71 81 71 71 80 76 74 77 ...
$ ChildMortality : num 16.7 3.2 16.4 4 35.2 5.2 4.2 6.7 12.1 4.7 ...
$ CellularSubscribers : num 96.4 75.5 103.6 154.8 108.8 ...
$ LiteracyRate : num NA NA 99.6 NA NA NA NA 97.9 NA 98.8 ...
$ GNI : num 8820 NA 6100 42050 8960 ...
$ PrimarySchoolEnrollmentMale : num NA 78.4 NA NA 85.3 NA 98.9 86.5 99.3 94.8 ...
$ PrimarySchoolEnrollmentFemale: num NA 79.4 NA NA 84.1 NA 99.2 88.4 99.7 97 ...
> Summary(WHO_Europe)
Error in (function (classes, fdef, mtable) :
unable to find an inherited method for function ‘Summary’ for signature ‘"data.frame"’
> write.csv(WHO_Europe,"WHO_Europe.CSV")
> Summary(WHO_Europe)
Error in (function (classes, fdef, mtable) :
unable to find an inherited method for function ‘Summary’ for signature ‘"data.frame"’