## 5 Discussion  
Our analysis revealed the extent to which green purchasing has been advanced in Japan by category or item. By observing in which categories or for which items the green purchasing rate is high, we can conjecture which characteristics help promote green purchasing by municipalities. At the same time, by identifying the categories or items that lag behind in green purchasing, we can propose policy measures that are useful for municipalities in terms of these items. We can summarize the descriptive statistics of our data as follows. Our data reveal the characteristics of items that are more likely to advance in green purchasing and show that items tend to progress in green purchasing if they are purchased more frequently and/or in larger quantities (e.g., paper products and stationery). One possible reason for this success is that purchasing officers can improve the implementation of green purchasing with experience. If the frequency of purchasing is higher, then the purchasing officer can learn how to implement green purchasing by learning by doing, as pointed out by previous studies [12; 37; 38; 39; 40; 41]. Alternatively, the cost of green purchasing may be relatively lower for such items. Therefore, even smaller municipalities can purchase the green products of such items [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. Moreover, our data show that if items are often purchased in bulk (e.g., uniforms, work gloves, paper, stationery, etc. ), then they also tend to be prioritized for green purchasing. One plausible reason is that bulk purchasing eliminates the need for interdepartmental coordination. In addition, green purchasing is also advanced in items for which criteria such as an \"ecolabel\" are used to identify eco-friendly goods [18; 19; 20; 21; 22; 23]. The opposite is true for several items, such as mobile phones. In contrast, tailored items such as public works and AC units are less likely to advance in green purchasing [18; 20; 21; 22; 23]. With regard to AC units, procurement officers have to specify the performance of the goods to meet the certain criteria of energy efficiency since no explicit standards exist for office AC units. This situation is also true for public works. For these items, it is more difficult for procurement officers to identify whether a product or contract is environmentally friendly. In addition, the green purchasing rate is less likely to be measured for tailored items. Next, our estimation results indicate that a GPP can improve the advancement of green purchasing in terms of both its implementation and measurement, even for items for which green purchasing is relatively difficult to implement. The results are also robust when controlling for the size of the municipality. We find that for all items, implementation of green purchasing is more advanced in municipalities with a GPP. In terms of such implementation, there are large differences between municipalities with and without a GPP for uniforms, work gloves, and computers, with differences exceeding 30%. Conversely, the percentage is approximately 20% for paper products and 13% for mobile phones. In other words, it is clear that the impact of a GPP is relatively high for items for which green purchasing as a whole is in progress to some extent. The same is true for the measurement of green purchasing. We also find that the impact of GPP adoption is higher on the measurement of green purchasing than on its implementation. One reason for the particularly large impact of GPP adoption on measurement might be that GPP adoption forces municipalities to manage green purchases, and measurement is necessary for such management. Another reason is that starting to measure the green purchasing rate is relatively easier than starting to implement green purchasing. The implementation of green purchasing requires coordination among departments making green purchases, which is particularly difficult in cases of separate purchases.