| The I period (2000-2004) | The II period (2005-2009) | The EI period (2010∼2014) | The IV period (2015∼2017) | ||||
|---|---|---|---|---|---|---|---|
| Keyword | TF-IDF | Keyword | TF-IDF | Keyword | TF-IDF | Keyword | TF-IDF |
| Exercise | 1107.65 | Ice rink | 651.56 | Famous and delicousor tasty food are sold | 1,351.93 | Cafe | 1,337.44 |
| Plaza | 609.28 | Picture | 609.07 | Outdoor | 1,012.34 | Picnic | 1,294.17 |
| Take a picture | 561.59 | PSY (Singer) | 548.97 | Date | 995.34 | Landscape | 1,139.48 |