digraph mygraph {
  node [shape=box];
  { rank = same; "二項分布"; "ポアソン分布" ;
  }
  
  "二項分布" 
  "二項分布" -> "ポアソン分布"  [label = "大標本, 低確率"]
  "二項分布" -> "正規分布"  [label = "大標本"]
  "ポアソン分布" -> "正規分布" [label = "頻度が大"]
  "正規分布" -> "Z 分布" [label = "頻度が大"]
{
 rank = same; "Z 分布" ; "カイ二乗分布"
}
  "Z 分布" -> "カイ二乗分布" [label = "平方和"]
  "カイ二乗分布" -> "Z 分布" [label = "大標本"]
  "Z 分布" -> "t 分布" [label = "母分散未知"]
{
 rank = same; "t 分布" ; "F 分布"
}
  "t 分布" -> "F 分布" [label = "二乗"]
  "カイ二乗分布" -> "F 分布" [label = "大標本"]
  label = "標本分布の関係";
}