while the mean (and median) share of cities that have a single center is 80 percent (rows 1 and 3 respectively). Moreover, in 90 percent of countries, 90 percent of cities have a single center (row 3).<sup>34</sup> Notwithstanding the dominance of monocentric cities, however, the above results still imply that, on average, one-fifth of an SSA country’s cities have multiple centers. For one-half of these cities, the number of centers is two (row 4, table 8). Thus, most cities are either monocentric or, if not, duo centric in terms of their spatial population distributions. The largest cities in SSA, however, have many centers. Hence, the mean number of centers in a country’s largest city is 46, while the median is 4 (row 7). The fact that the mean is far higher than the median implies there is a strong positive skew to the distribution with a few countries having largest cities with very many centers. Thus, for 10 percent of SSA countries, the largest city has a remarkable 195 or more centers. As per the discussion above, many of these “centers” are likely to be densely populated slum areas, many of which may be quite small. As Table A3 in the Annex shows, Kenya, Ethiopia, and Niger are the three countries whose largest cities (Nairobi, Addis Ababa, and Niamey respectively) have the most centers.<sup>35</sup>

**<u>Table 8: Distribution of city center characteristics over 43 SSA countries</u>**

|||**Mean**|**Std. Error**|**P10**|**P25**|**P50**|**P75**|**P90**|
|---|---|---|---|---|---|---|---|---|
|1|No. centers|1.65|0.58|1.11|1.21|1.51|1.81|2.22|
|2|Sh. 0 cen.|0.01|0.02|0|0|0|0.02|0.05|
|3|Sh. 1 cen.|0.8|0.09|0.69|0.74|0.8|0.85|0.9|
|4|Sh. 2 cen.|0.1|0.05|0.05|0.07|0.1|0.13|0.16|
|5|Sh. 3-5 cen.|0.06|0.04|0.01|0.03|0.06|0.08|0.13|
|6|Sh.>6 cen.|0.03|0.03|0|0|0.02|0.04|0.05|
|7|#cen. largest|45.98|112.29|1|2|4|14|195|
|8|Sh. pop. lar. cen.|0.68|0.08|0.57|0.61|0.67|0.73|0.77|
|9|Sh. bui. lar. cen.|0.57|0.07|0.49|0.52|0.57|0.62|0.65|
|10|Sh. area lar. cen.|0.26|0.08|0.21|0.22|0.24|0.27|0.38|
|11|lar/2nd pop.|12.86|15.79|3.96|5.24|6.54|12.37|35.29|
|12|lar/2nd bui.|1852.1|9456.56|13.56|25.76|54.98|331.01|1405.52|
|13|lar/2nd area|11.25|14.19|3.97|5.05|6.32|10.96|27.81|
|14|Zipf centers|-0.57|0.12|-0.72|-0.67|-0.58|-0.5|-0.4|
|15|Elas. #cen. pop.|0.2|0.12|0.04|0.1|0.17|0.29|0.35|

**Notes:** _No. centers:_ Number of centers per city. _Sh. 0 cen.:_ Share of cities without center. _Sh. 1 cen.:_ Share of cities with one center. _Sh. 2 cen.:_ Share of cities with two centers. _Sh. 2-3 cen.:_ Share of cities with two to five centers. _Sh. >6 cen.:_ Share of cities with 6 or more centers. _#cen. largest:_ Number of centers of the largest city. _Sh. pop. lar. cen.:_ City population share of the largest center. _Sh. bui. lar. cen.:_ City built share of the largest center. _Sh. area lar. cen.:_ City area share of the largest center. _lar/2nd pop.:_ Relative population of the largest over the second largest center. _lar/2nd bui.:_ Relative built of the largest over the second largest center. _lar/2nd area:_ Relative area of the largest over the second largest center. _Zipf centers:_ Within city population Zipf's coefficient for city centers. _Elas. #cen. pop.:_ elasticity of the number of centers in the city with respect to its population. Statistics are computed over the crosssection of 43 SSA countries, Comoros, Madagascar, Seychelles and São Tomé Príncipe being excluded for the withincity statistics given their to low number of cities or of centers in these cities. PX corresponds to the X<sup>th</sup> percentile of the <u>distribution.</u>

On average across countries, a city’s largest, and, typically, only, center, occupies 26 percent of its land area but accounts for 57 percent of its built-up density and 68 percent of its population (rows 8-10, Table 8). This is consistent with the fact that, by definition, centers are contiguous sets of grid cells with very high population densities relative to all other cells within a city. For cities that have multiple centers, the largest center is typically much larger than the second largest center. Thus, across countries, the population of a city’s largest center is, on average, almost 13 times that of its second largest center, while the area occupied by a city’s largest center is, on average, more than 11 times that occupied by its second largest center (rows

> 34 As shown in row 2 of table 8, there is a very small minority of cities that have no statistically identifiable centers. A city can have no statistically significant population density peak if its population is very evenly distributed across its extent.

> 35 Even though the largest cities have many centers, for 72 percent of SSA countries, the city with the most centers is, somewhat surprisingly, _not_ its largest city. And, across countries, the mean number of centers in the city with the most centers is 61.

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