Why the "max" is ambiguous occur?

#include<iostream>
#include<iomanip>
#include<cmath>

using namespace std;
//function prototype
int AVSD(double[], double&, double&, const int);
int max;	

int main() {
	const int SIZE = 10;
	int index = 0;
	double average = 0, SD = 0;
	double studyHour[SIZE] = {10,16,15.65,17,16.3,20,10.5,17.5,12 };

	index = AVSD(studyHour, average, SD, SIZE);
	cout << "Details of Practical Group 1" << endl;
	cout << "Average:" << average<<endl;
	cout << "Standard Deviation:" <<SD<< endl;
	cout << "Highest study hour" <<studyHour[max]<<endl;
	system("PAUSE");
	return 0;
}

int AVSD(double studyHour[], double& average, double& SD, const int SIZE) {

	double sum = 0;
	for (int i = 0; i < SIZE; i++) {
		sum = sum + studyHour[i];
		average = sum / SIZE;
	}

	double total = 0;
	for (int i = 0; i < SIZE; i++) {
		pow((studyHour[i] - average), 2);
		total += pow((studyHour[i] - average), 2);
	}
	SD = sqrt(fabs(total / SIZE));

	for (int i = 0; i < 10; i++) {
		if (i == 0) {
			max = i;
		}
		else {
			if (studyHour[i] > studyHour[max])
				max = i;
		}
return max;
	}
}