  Procedures are inherently hierarchical. To "make videos", one may need to
"purchase a camera", which in turn may require one to "set a budget". While
such hierarchical knowledge is critical for reasoning about complex procedures,
most existing work has treated procedures as shallow structures without
modeling the parent-child relation. In this work, we attempt to construct an
open-domain hierarchical knowledge-base (KB) of procedures based on wikiHow, a
website containing more than 110k instructional articles, each documenting the
steps to carry out a complex procedure. To this end, we develop a simple and
efficient method that links steps (e.g., "purchase a camera") in an article to
other articles with similar goals (e.g., "how to choose a camera"), recursively
constructing the KB. Our method significantly outperforms several strong
baselines according to automatic evaluation, human judgment, and application to
downstream tasks such as instructional video retrieval.
  A demo with partial data can be found at https://wikihow-hierarchy.github.io.
The code and the data are at https://github.com/shuyanzhou/wikihow_hierarchy.
